U.S. Department of the Interior

U.S. Geological Survey

Circular 1359

Groundwater Quality in the Columbia Plateau and 

Snake River Plain Basin-Fill and Basaltic-Rock Aquifers 

and the Hawaiian Volcanic-Rock Aquifers, Washington, 

Idaho, and Hawaii, 1993–2005

The Quality of Our Nation’s Waters

National Water-Quality Assessment Program
Cover photographs.

Background: Wheat harvest on the Palouse hills of the Columbia Plateau, Washington (United States Department of Agriculture, 

Agricultural Research Service).

Foreground, left: Aerial photograph of a dairy in the Snake River Plain, Idaho (copyright Kestrel Aerial Services, Inc., 

©www.kestrelaerial.com, used with permission).

Foreground, right: Looking north at Honolulu, Hawaii, from the top of Diamondhead (Michael G. Rupert, U.S. Geological Survey). 
The Quality of Our Nation’s Waters 

Groundwater Quality in the Columbia Plateau and 

Snake River Plain Basin-Fill and Basaltic-Rock Aquifers 

and the Hawaiian Volcanic-Rock Aquifers, Washington, 

Idaho, and Hawaii, 1993–2005

By Michael G. Rupert, Charles D. Hunt, Jr., Kenneth D. Skinner, Lonna M. Frans, and Barbara J. Mahler

National Water-Quality Assessment Program

Circular 1359

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Rupert, M.G., Hunt, C.D., Jr., Skinner, K.D., Frans, L.M., and Mahler, B.J., 2014, The quality of our Nation’s waters—

Groundwater quality in the Columbia Plateau and Snake River Plain basin-fill and basaltic-rock aquifers and the 

Hawaiian volcanic-rock aquifers, Washington, Idaho, and Hawaii, 1993–2005: U.S. Geological Survey Circular 1359,

88 p., http://dx.doi.org/10.3133/cir1359.

Library of Congress Cataloging-in-Publication Data

Rupert, Michael G., 1959-

 The quality of our nation’s waters---groundwater quality in the Columbia Plateau and Snake River Plain basin-fill 

and basaltic-rock aquifers and the Hawaiian volcanic-rock aquifers, Washington, Idaho, and Hawaii, 1993--2005 / by

Michael G. Rupert, Charles D. Hunt, Jr., Kenneth D. Skinner, Lonna M. Frans, and Barbara J. Mahler.

 pages cm. -- (Circular / U.S. Geological Survey ; 1359)

 Includes bibliographical references.

 ISBN 978-1-4113-3757-2

1. Water quality--Columbia Plateau. 2. Water quality--Idaho--Snake River Plain Aquifer. 3. Water quality--

Washington (State) 4. Water quality--Hawaii. 5. Groundwater--Quality--Columbia Plateau. 6. Groundwater--Quality--

Idaho--Snake River Plain Aquifer. 7. Groundwater--Quality--Washington (State) 8. Groundwater--Quality--Hawaii. 

I. Hunt, Charles D. II. Skinner, K. D. (Kenneth D.) III. Frans, Lonna M. IV. Mahler, B. J. (Barbara June), 1959- 

V. Geological Survey (U.S.) VI. Title. VII. Series: U.S. Geological Survey circular; 1359.

 TD223.7.R87 2014

 628.1’1409797--dc23

2014001749

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

ISBN 978-1-4113-3757-2
Photograph by Michael G. Rupert, USGS

iii

National Water-Quality Assessment Program

“The NAWQA study results provide Hawaii with an important 

database describing the status of water quality in aquifers on Oahu. 

Many of Oahu’s groundwaters have been subject to only limited 

monitoring in the past, so these data will represent a baseline to 

which results from future studies can be compared. Reports on 

chemical contaminants in aquifers are particularly useful because 

these data are expensive to obtain and thus rarely collected. Similar 

data sets from the other Hawaiian islands are sorely needed.”

—Dr. June Harrigan, Hawaii Department of Health, Environmental 

Planning Office
iv

Foreword

The United States has made major investments in assessing, managing, regulating, and conserving 

natural resources, such as water and a variety of ecosystems. Sustaining the quality of the 

Nation’s water resources and the health of our diverse ecosystems depends on the availability 

of sound water-resources data and information to develop effective, science-based policies. 

Effective management of water resources also brings more certainty and efficiency to important 

economic sectors. Taken together, these actions lead to immediate and long-term economic, 

social, and environmental benefits that make a difference to the lives of millions of people 

(http://water.usgs.gov/nawqa/applications/).

Two decades ago, Congress established the U.S. Geological Survey’s National Water-Quality 

Assessment (NAWQA) Program to meet this need. Since then NAWQA has served as a primary 

source of nationally consistent information on the quality of the Nation’s streams and groundwater, 

on ways in which water quality changes over time, and on the natural features and human activities 

affecting the quality of streams and groundwater. Objective and reliable data, systematic scientific 

studies, and models are used to characterize where, when, and why the Nation’s water quality 

is degraded—and what can be done to improve and protect the water for human and ecosystem 

needs. This information is critical to our future because the Nation faces an increasingly complex 

and growing need for clean water to support people, economic growth, and healthy ecosystems. For 

example, NAWQA findings for public-supply wells, which provide water to about 105 million people, 

showed that 22 percent of source-water samples contained at least one contaminant at levels of 

potential health concern. Similarly, 23 percent of samples from domestic (or privately owned) wells, 

which supply untreated water to an additional 43 million people, also had contaminant levels of 

potential concern.

This report is one of a collection of publications that describe water-quality conditions in selected 

Principal Aquifers of the United States (http://water.usgs.gov/nawqa/studies/praq/). The collection 

is part of the series “The Quality of Our Nation’s Waters,” which describes major findings of the 

NAWQA Program on water-quality issues of regional and national concern and which provides 

science-based information for assessing and managing the quality of our groundwater resources. 

Other reports in this series focus on occurrence and distribution of nutrients, pesticides, and volatile 

organic compounds in streams and groundwater, the effects of contaminants and streamflow 

alteration on the condition of aquatic communities in streams, and the quality of untreated water 

from private domestic and public-supply wells. Each report builds toward a more comprehensive 

understanding of the quality of regional and national water resources (http://water.usgs.gov/nawqa/

nawqa_sumr.html). All NAWQA reports are available online at http://water.usgs.gov/nawqa/bib/.

The information in this series primarily is intended for those interested or involved in resource 

management and protection, conservation, regulation, and policymaking at regional and national 

levels. In addition, the information should be of interest to those at a local level who wish to know 

more about the general quality of streams and groundwater in areas near where they live and how 

that quality compares with other areas across the Nation. We hope this publication will provide you 

with insights and information to meet your needs and will foster increased citizen awareness and 

involvement in the protection and restoration of our Nation’s waters.

Jerad Bales 

Acting Associate Director for Water

U.S. Geological Survey
http://pubs.usgs.gov/sir/2014/1360

v

California Coastal Basin aquifers

Central Valley aquifer system

Basin and Range basin-fill aquifers

Rio Grande aquifer system

Snake River Plain basin-fill and 

basaltic-rock aquifers

Columbia Plateau basin-fill and

basaltic-rock aquifers 

Mississippi embayment–Texas coastal

uplands aquifer system—Circular 1356

Mississippi River Valley alluvial aquifer

Denver Basin aquifer system—Circular 1357

Piedmont and Blue Ridge 

carbonate-rock aquifers

Piedmont and Blue Ridge 

crystalline-rock aquifers

Valley and Ridge carbonaterock aquifers

High Plains aquifer system—Circular 1337

Upper Floridan aquifer and overlying

surficial aquifers—Circular 1355

Northern Atlantic Coastal Plain 

surficial aquifer system—Circular 1353

Glacial aquifer system—Circular 1352

Hawaiian volcanic-rock aquifers

Early Mesozoic basin aquifers

Valley and Ridge siliciclasticrock aquifers

Southwest Principal Aquifers—Circular 1358 Piedmont, Blue Ridge, and Valley and 

Ridge aquifers—Circular 1354

Western Volcanics—Circular 1359

Principal Aquifers and Water-Quality Summary Reports

National summary of groundwater quality in Principal Aquifers—Circular 1360

High Plains aquifer system—Circular 1337

Glacial aquifer system—Circular 1352

Northern Atlantic Coastal Plain surficial

aquifer system—Circular 1353

Piedmont, Blue Ridge, and Valley and 

Ridge aquifers—Circular 1354

Piedmont and Blue Ridge 

carbonate-rock aquifers

Piedmont and Blue Ridge 

crystalline-rock aquifers

Valley and Ridge siliciclasticrock aquifers

Valley and Ridge carbonaterock aquifers

Early Mesozoic basin aquifers 

Upper Floridan aquifer and overlying

surficial aquifers—Circular 1355

Mississippi embayment–Texas coastal 

uplands aquifer system—Circular 1356

Mississippi River Valley alluvial aquifer

Denver Basin aquifer system—Circular 1357

Southwest Principal Aquifers—Circular 1358

California Coastal Basin aquifers

Central Valley aquifer system

Basin and Range basin-fill aquifers

Rio Grande aquifer system 

Western Volcanics—Circular 1359

Hawaiian volcanic-rock aquifers

Snake River Plain basin-fill and 

basaltic-rock aquifers

Columbia Plateau basin-fill and 

basaltic-rock aquifers
vi

Contents

Chapter 1: Overview of Major Findings and Implications ......................................................1

Chapter 2: NAWQA Approach to Assessing Groundwater Quality ......................................7

Chapter 3: Environmental and Hydrogeologic Setting .........................................................15

Chapter 4: Natural Processes and Human Activities That Affect 

Groundwater Quality ...............................................................................................27

Chapter 5: Quality of the Groundwater Resource Used for Drinking .................................33

Chapter 6: Understanding Where and Why Key Contaminants Occur

in Groundwater........................................................................................................39

References ...............................................................................................................................67

Glossary ...............................................................................................................................73

Appendixes ...............................................................................................................................79
vii

Introduction to This Report

This report contains the major findings of a regional assessment of groundwater quality in three 

areas: the Columbia Plateau in Washington, the Snake River Plain in Idaho, and the island of Oahu, 

Hawaii. It is one of a series of reports by the U.S. Geological Survey (USGS) National Water-Quality 

Assessment (NAWQA) Program that present major findings for Principal Aquifers, other aquifers, and 

major river basins across the Nation. In these reports, water quality is discussed in terms of local, 

State, regional, and national issues. Conditions in the Columbia Plateau, the Snake River Plain, and 

Oahu are compared to conditions found elsewhere and to selected national benchmarks, such as 

those for drinking-water quality.

This report is intended for individuals working with water-resource issues in local, State, or Federal 

agencies, universities, public interest groups, or the private sector. The information will be useful in 

addressing current issues, such as drinking-water quality, source-water protection, and monitoring 

and sampling strategies. This report will be useful for individuals who wish to know more about the 

quality of groundwater in areas near where they live and how that quality of water compares to that 

in other areas across the region and the Nation.

Water-quality conditions in the Columbia Plateau, the Snake River Plain, and Oahu summarized in this 

report are discussed in greater detail in other reports listed in the references. Detailed technical information, data and analyses, sample collection and analytical methodology, models, graphs, and maps 

that support the findings presented in this report in addition to reports in this series from other Principal Aquifers can be accessed from the national NAWQA Web site (http://water.usgs.gov/nawqa/).

Companion studies of the groundwater quality in the Columbia Plateau and Snake River Plain basin-fill 

and basaltic-rock aquifers and Hawaiian volcanic-rock aquifers are discussed in the following reports:

Numerical Model Simulations of Nitrate Concentrations in 

Groundwater Using Various Nitrogen Input Scenarios, 

Mid-Snake Region, South-Central Idaho

U.S. Geological Survey Scientific Investigations Report 2012–5237

By Kenneth D. Skinner and Michael G. Rupert

(Also available at http://pubs.usgs.gov/sir/2012/5237/)

Groundwater Quality in the Columbia Plateau, Snake River Plain, 

and Oahu Basaltic-Rock and Basin-Fill Aquifers in the 

Northwestern United States and Hawaii, 1992–2010

U.S. Geological Survey Scientific Investigations Report 2012–5123

By Lonna M. Frans, Michael G. Rupert, Charles D. Hunt, Jr., and 

Kenneth D. Skinner

(Also available at http://pubs.usgs.gov/sir/2012/5123/)
viii

Water Quality in the Central Columbia Plateau,

Washington and Idaho, 1992– 95

U.S. Geological Survey Circular 1144

By Alex K. Williamson, Mark D. Munn, Sarah J. Ryker, 

Richard J. Wagner, James C. Ebbert, and Ann M. Vanderpool

(Also available at http://pubs.usgs.gov/circ/circ1144/)

Water Quality in the Upper Snake River Basin, 

Idaho and Wyoming, 1992– 95

U.S. Geological Survey Circular 1160

By G.M. Clark, T.R. Maret, M.G. Rupert, M.A. Maupin, 

W.H. Low, and D.S. Ott

(Also available at http://pubs.usgs.gov/circ/circ1160/)

Water Quality on the Island of Oahu, Hawaii, 1999 – 2001

U.S. Geological Survey Circular 1239

By Stephen S. Anthony, Charles D. Hunt, Jr., Anne M.D. Brasher, 

Lisa D. Miller, and Michael S. Tomlinson

(Also available at http://pubs.usgs.gov/circ/2004/1239/)
Chapter 1

OREGON

WASHINGTON

NEVADA

ARIZONA NEW

MEXICO

COLORADO

MONTANA

IDAHO

WYOMING

UTAH CALIFORNIA

HAWAII

Residents of the Columbia Plateau, the Snake River Plain, and the island 

of Oahu depend on groundwater as their primary source of drinking 

water. Although the depth to the water table can be several hundred 

feet, the groundwater is vulnerable to contamination because the permeable 

sediments and rocks in these areas allow contaminants to move readily down 

to the water table. Intense agricultural activities overlie groundwater used for 

drinking-water supply and these activities are increasing in some areas. For 

example, the number of dairy cattle in the Snake River Plain of Idaho increased 

fivefold between 1987 and 2007, an increase that is roughly equivalent in terms 

of nitrogen production to a population growth of more than 4 million people. 

Nitrogen and other contaminants associated with such agricultural activities 

can adversely affect groundwater quality. In Hawaii, which is often thought of 

as a pristine tropical environment, past use of agricultural chemicals to control 

insect pests has led to persistent groundwater contamination in some areas. 

Groundwater has become an

 increasingly important source for

agricultural and domestic water needs. 

Photograph © 1980 by W.H. Mullins 

and published with permission

Chapter 1: Overview of Major Findings 

and Implications
Forest

Saturated

Perched

Unsaturated

Deep water table

Irrigated row crops

Perched water table

Urban, military

Regional groundwater flow 

Domestic

well

Public-supply

well

Monitoring

well

Monitoring well

Perched water body

Unsaturated basalt

Deep water body

Centerfold.

The Columbia Plateau, Snake River Plain, 

and Oahu aquifers are vulnerable to 

contaminants related to human activities 

despite depths to groundwater that are 

among the greatest in the Nation

1

Although depth to the water table can be many 

hundreds of feet, the permeable geologic materials 

overlying the aquifers allow many contaminants to 

move readily from the land surface to the water table. 

For example, nitrate concentrations were above the 

drinking-water standard (U.S. Environmental Protection 

Agency Maximum Contaminant Level, USEPA MCL) 

of 10 milligrams per liter (mg/L) as nitrogen in water 

from 20 percent of wells sampled in the Columbia 

Plateau, and 4 percent of wells sampled in the Snake 

River Plain. One or more pesticide compounds were 

detected in water from half of the wells sampled in all 

three areas combined, though most concentrations were 

below human-health benchmarks.

Groundwater 

in the Columbia 

Plateau had 

higher nitrate 

concentrations 

than did groundwater in the 

Snake River Plain 

or on Oahu.

Concentration, in

milligrams per liter as nitrogen

Background

(1 mg/L)

40

35

30

25

20

15

10

5

0

Outlier at

58 mg/L 

not shown

Outlier at

59 mg/L 

not shown

Nitrate concentration

USEPA MCL (10 mg/L)

Columbia 

Plateau

Snake River 

Plain

Oahu

Foldout

See sidebar, Boxplots, p. 41

Overview of Major Findings of Groundwater Quality for the Columbia 

and the Hawaiian Volcanic-Rock Aquifers

The Columbia Plateau, Snake River Plain, and Hawaii are large 

volcanic areas in the western United States and mid-Pacific ocean that 

contain extensive regional aquifers of a hard, gray, volcanic rock called 

basalt. Residents of the Columbia Plateau, the Snake River Plain, and the 

island of Oahu depend on groundwater as their primary source of drinking 

water. Although the depth to the water table can be several hundred feet, the 

groundwater is highly vulnerable to contamination because the permeable 

sediments and rocks allow contaminants to move readily down to the water 

table. Intense agricultural and urban activities occur above the drinkingwater supply and are increasing in some areas. Contaminants such as nitrate, 

pesticides, and volatile organic compounds, associated with agricultural 

and urban activities, have adversely affected groundwater quality. 

1

2
Unweathered

basalt

NOT TO SCALE

Weathered basalt

(saprolite)

Soil

Urban

Publicsupply

well

Irrigation

well

Chemicals that have been banned for over 30 years are still a potential human-health 

concern in groundwater

Although the aquifers of the Columbia Plateau, Snake 

River Plain, and Oahu are composed of similar geologic 

materials (basalt), contamination issues vary among the 

aquifers because of differing geochemical conditions 

and differing land and chemical uses

2

3

Several soil fumigants and the pesticide dieldrin were banned more 

than 30 years ago, yet have been detected at concentrations exceeding 

human-health benchmarks in some groundwater samples. Chemicals 

that are no longer in use but are persistent in the environment are termed 

“legacy contaminants.” These chemicals do not readily break down in 

groundwater, and several decades (or longer) are required to flush the 

chemicals through the aquifers. Exceedances of human-health benchmarks occurred most frequently on Oahu, where concentrations of one 

or more soil fumigants exceeded Hawaii MCLs in water from 13 percent 

of public-supply wells sampled, and concentrations of dieldrin exceeded 

the human-health benchmark in water from 10 percent of public-supply 

wells sampled. Tracking changes in concentrations of legacy contaminants could require decades of groundwater monitoring.

Contaminants from geologic sources and nitrate were more 

commonly detected in groundwater sampled in the Columbia 

Plateau and Snake River Plain than on Oahu. For example, elevated 

concentrations of radon, arsenic, molybdenum, and manganese were 

detected in water from some wells in the Columbia Plateau and Snake 

River Plain, but not in wells on Oahu, because the aquifer materials 

and geochemical conditions on Oahu do not promote 

mobilization of these constituents in groundwater. 

Contaminants related to human activities—soil 

fumigants and solvents—were detected more frequently 

on Oahu than in the Columbia Plateau and Snake River 

Plain. At least one soil fumigant or solvent was detected 

in water from more than half of the wells sampled on 

Oahu. These contaminants are present in groundwater 

on Oahu because of past intensive application of soil 

fumigants to control insect pests in a tropical climate 

and releases of solvents in military and urban lands. The 

discovery of soil fumigant and solvent contamination on 

Oahu in the 1980s led to well closures and installation of 

costly water-treatment facilities.

Photograph by Peggy Greb, USDA ARS

Plateau and Snake River Plain Basin-Fill and Basaltic-Rock Aquifers 

Water-treatment systems are required to 

remove soil fumigants from drinking water 

on Oahu.

Soil fumigants are used in 

pineapple cultivation.

Photograph courtesy of GMP International

3
1 The Columbia Plateau, Snake River Plain, and Oahu aquifers 

are vulnerable to contaminants related to human activities despite

depths to groundwater that are among the greatest in the Nation. 

Nitrite plus nitrate as nitrogen (nitrate) and pesticides were widely detected in groundwater samples from the Columbia Plateau and the Snake River Plain aquifers. Although 

depth to the water table can be many hundreds of feet, the permeable geologic materials 

overlying the aquifers allow contaminants to move readily from the land surface to the water 

table. Human activities associated with intensive agriculture in areas that rely on aquifers 

for drinking water are a source of numerous contaminants, including nitrate and pesticides. 

For example, nitrate concentrations were above the U.S. Environmental Protection Agency 

Maximum Contaminant Level of 10 milligrams per liter in water from 20 percent of wells 

sampled in the Columbia Plateau, and 4 percent of wells sampled in the Snake River 

Plain. One or more pesticide compounds were detected in water from half of the wells 

sampled in the three areas combined, though most concentrations were below humanhealth benchmarks. The widespread occurrence of pesticides and elevated concentrations 

of nitrate demonstrates the vulnerability of the aquifers to contamination and indicates 

that comprehensive groundwater protection strategies are warranted to help assure those 

concentrations do not exceed human-health benchmarks in the future. 

2 Chemicals that have been banned for over 30 years are still 

a potential human-health concern in groundwater.

Chemicals that are no longer in use but are persistent in the environment are termed 

“legacy contaminants.” These chemicals include several soil fumigants and the pesticide 

dieldrin, which were banned more than 30 years ago, yet have been detected at concentrations 

exceeding human-health benchmarks in some groundwater samples. Exceedances of humanhealth benchmarks occurred most frequently on Oahu, where concentrations of one or more 

soil fumigants exceeded Hawaii Maximum Contaminant Levels in water from 13 percent 

of public-supply wells sampled, and concentrations of dieldrin exceeded the human-health 

benchmark in water from 10 percent of public-supply wells sampled. Soil fumigants also 

exceeded human-health benchmarks in groundwater samples from the Columbia Plateau 

and Snake River Plain, but in only a few wells. These chemicals do not readily break down 

in groundwater, and several decades (or longer) are required to flush the chemicals through 

the aquifers. For these reasons, current land use may not be a good indicator of existing 

groundwater quality because contamination from former land use and chemical applications 

may be present. As an example, if existing agricultural land is converted to residential use, 

homeowners and developers could encounter groundwater contaminated with agricultural 

legacy contaminants. This was the case on Oahu, where public-supply wells drilled in 

former agricultural lands tapped groundwater contaminated by fumigants, requiring 

installation of expensive treatment systems. Tracking changes in concentrations of legacy 

contaminants could require decades of groundwater monitoring. 

Photographs from top to bottom: USDA ARS; Keith Weller, 

USDA ARS; Michael G. Rupert, USGS; Michael G. Rupert, USGS

4 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005

Overview of Major Findings of 

and Snake River Plain Basin-Fill and Basaltic-Rock 
Chapter 1

3 Although the aquifers of the Columbia Plateau, Snake River 

Plain, and Oahu are composed of similar geologic materials 

(basalt), contamination issues vary among the aquifers 

because of differing geochemical conditions and differing 

land and chemical uses.

Contaminants from geologic sources and nitrate were more commonly detected in 

groundwater sampled in the Columbia Plateau and Snake River Plain than on Oahu, but 

contaminants related to human activities were more commonly detected in groundwater 

on Oahu. For example, elevated concentrations of radon, arsenic, molybdenum, and 

manganese were detected in water from some wells in the Columbia Plateau and Snake 

River Plain, but not in wells on Oahu, because the aquifer materials and geochemical 

conditions on Oahu do not promote mobilization of these constituents in groundwater. 

Groundwater in the Columbia Plateau and Snake River Plain had much higher 

concentrations of nitrate than did groundwater on Oahu—nitrate concentrations in 

Columbia Plateau water samples were about 5 times higher than in samples from Oahu. 

Nitrate concentrations in groundwater in the Snake River Plain are rising, reflecting a 

near doubling of nitrogen input from nitrogen fertilizer and cattle manure between 1987 

and 2007. Computer modeling forecasts that, if nitrogen inputs remain at current (2008) 

levels, nitrate concentrations in Snake River Plain groundwater will continue to increase 

for at least 50 years; therefore, agricultural management practices that decrease nitrogen 

application will be all the more important in the coming decades. Continued public 

outreach and education can increase homeowners’ awareness of the need for domesticwell testing and of the water-treatment options available.

Soil fumigants and solvents, contaminants related to human activities, were detected 

much less frequently in the Columbia Plateau and Snake River Plain than on Oahu. 

The image of Oahu is that of a pristine tropical environment, but groundwater in many 

areas of the island is contaminated with soil fumigants and solvents, in some cases at 

concentrations of concern for human health. At least one soil fumigant or solvent was 

detected in water from more than half of the wells sampled on Oahu. These contaminants 

are present in groundwater on Oahu because of past intensive application of soil 

fumigants to control insect pests in a tropical climate and releases of solvents in military 

and urban lands. Discovery of soil fumigant and solvent contamination on Oahu in the 

1980s led to well closures and installation of costly water-treatment facilities. Wellhead 

protection programs can minimize future groundwater contamination related to human 

activities, but the decades-long persistence of contaminants from former chemical 

releases and application may continue to place a financial burden on society in the form 

of water-treatment costs.

Photographs from top to bottom: Doug Peebles, Douglas Peebles Photography; 

Scott Bauer, USDA ARS; Scott Bauer, USDA ARS; David Nance, USDA ARS

Chapter 1: Overview of Major Findings and Implications 5

Groundwater Quality for the Columbia Plateau 

Aquifers and the Hawaiian Volcanic-Rock Aquifers
Taggart Creek, in the Jackson valley study area, near 

Grand Teton National Park, Wyoming. Creeks such as 

this provide important recharge to groundwater.

Photograph by Michael G. Rupert, USGS
Chapter 2

This chapter summarizes 

the study design used to

investigate water quality

in the Columbia Plateau,

Snake River Plain, and 

Oahu aquifers.

As part of the NAWQA Program, water 

samples were collected from domestic, 

public-supply, monitoring, irrigation, and 

stock wells in a variety of settings. Here, 

horses graze next to a domestic well in 

the Jackson valley study area, Wyoming.

Photograph by Michael G. Rupert, USGS

Chapter 2: NAWQA Approach to Assessing 

Groundwater Quality

How does one go about characterizing the groundwater encompassing 

an area as large as that covered by the Columbia Plateau, Snake River 

Plain, and Oahu aquifers, let alone the entire United States? The 

approach taken by the USGS is to use different types of groundwater studies to 

answer the following questions: How does land use affect groundwater quality? 

How does water quality change as it moves through the aquifer? What is the 

quality of the drinking-water resource? Groundwater studies are the building 

blocks of NAWQA’s water-quality assessments of Principal Aquifers. Using a 

nationally consistent approach allows comparison between the many regions of 

the United States (see sidebar, Understanding study results, p. 9).
Mixed land uses 

(major aquifer study)

Special study

Agricultural lands 

(land-use study)

EXPLANATION

Well sampling groundwater underlying 

mixed land uses 

Well sampling groundwater underlying 

agricultural lands

Well sampling groundwater for 

special study

Figure 2-1.

OREGON IDAHO

WASHINGTON

Columbia

Plateau

Characterizing Groundwater Quality: 

The NAWQA Approach

Three types of studies were conducted in each Principal 

Aquifer, each with a different focus on information needs 

about groundwater quality and the natural and human-related 

factors that influence it. Groundwater studies designed to 

broadly assess water-quality conditions of the groundwater 

resource in relatively large areas were conducted to sample 

networks of existing domestic (private) and public-supply 

wells—these studies, called major aquifer studies or study 

unit surveys, involved sampling wells in areas with a mix of 

land uses (see appendix 1). Groundwater studies designed 

to characterize and explain the quality of recently recharged 

groundwater (generally less than 10 years old(1)) underlying 

agricultural and urban land were conducted to sample 

networks of domestic wells and shallow monitoring wells that 

were installed as part of the NAWQA Program—these studies 

are called land-use studies. Land-use studies were done in 

agricultural areas of the Columbia Plateau and Snake River 

Plain but not on Oahu. Urban areas were not studied in any 

of the three study areas. Groundwater studies that focused 

on areas of the aquifers where particular contaminants were 

expected to be higher than in other areas were conducted 

to sample domestic and monitoring wells—these studies 

are called special studies. Special studies were done in the 

Columbia Plateau and on Oahu but not in the Snake River 

Plain. The area sampled by the special studies was within the 

area sampled by the studies in agricultural lands, which in turn 

was within the area sampled in mixed land uses, providing 

a multiscale approach to understanding groundwater quality 

(fig. 2–1). 

Figure 2–1. The NAWQA approach in the Columbia Plateau and Snake River Plain and on Oahu used three types of groundwater 

studies (studies in areas of mixed land uses, studies in agricultural lands, and special studies) to assess groundwater-quality 

conditions at regional and local scales. The area sampled by the special studies was within the area sampled by the studies in 

agricultural lands, which in turn was within the area sampled by the studies in mixed land uses. Shown here is an example of the 

approach used in the Columbia Plateau.

8 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005 Alan M. Cressler, USGS Keith Weller, USDA ARS Scott Bauer, USDA ARS
Understanding study results

Important aspects of the NAWQA Principal Aquifer assessments:

•	 Water samples were collected at the wellhead (see sidebar, Anatomy of a well, p. 10) prior to any treatment. They 

represent the quality of the groundwater resource but not necessarily the quality of tap water. 

•	 The focus of the assessments is the condition of the total resource, including groundwater in a wide range of 

hydrologic and land-use settings across the Nation, rather than conditions at specific sites with known waterquality concerns.

•	 The assessments are guided by a nationally consistent study design, and all assessments use the same methods of 

sampling and analysis. Findings apply to water quality of a particular aquifer but also contribute to the larger picture 

of how and why water quality varies regionally and nationally. This consistent approach helps to determine if a waterquality issue is isolated or widespread. (See http://water.usgs.gov/nawqa/about.html for more information.)

•	 The assessments focus on aquifers used for water supply or on shallow groundwater that underlies an area with 

a particular type of land use. The wells sampled were selected to represent typical conditions in these aquifers. 

Because the wells are located only in NAWQA groundwater study areas, the samples may not be representative of 

all groundwater throughout the Principal Aquifers.

•	 Analytical methods used by USGS chemists for assessments of water quality in Principal Aquifers are designed to 

measure constituents at as low a concentration as possible (see appendix 2). As a result, constituents frequently 

are detected at concentrations far below human-health benchmarks for drinking water (see sidebar, Human-health 

benchmarks and other guidelines used in this assessment, p. 36). Low-level detections allow scientists to identify and 

evaluate emerging issues and to track contaminant levels over time. 

Is there a difference in the quality of water produced by domestic, public-supply, and 

monitoring wells?

The sampling results from domestic, public-supply, and monitoring wells were combined to provide a general overview of 

the quality of groundwater in the Columbia Plateau and Snake River Plain and on Oahu. But is there a difference in the quality 

of water produced by these three types of wells?

Domestic wells usually are shallower than public-supply wells; therefore, domestic wells pump water that is nearer 

to sources of manmade contaminants, such as fertilizers and pesticides, applied at the land surface. On the other hand, 

because public-supply wells pump water from deeper in the aquifer, they might be more likely to have higher concentrations 

of constituents from geologic sources, because deeper water generally has been in contact with the aquifer rock for a longer 

period of time. In addition to being deeper, public-supply wells have larger pumps and longer screened intervals than do 

domestic wells, and are pumped for longer periods of time. As a result, public-supply wells pump much larger volumes of 

water than do domestic wells, and so have much larger capture zones. Thus, public-supply wells are more vulnerable than 

are domestic wells to manmade contamination from distant sources. If the amount of water withdrawn is large enough, it 

can change the flow direction and velocity of the groundwater, which can, in turn, affect the groundwater chemistry and the 

constituents contained therein.

Domestic wells tend to be located in rural areas, so they are more likely to be vulnerable to contamination from agricultural chemicals, whereas public-supply wells, which tend to be located in suburban and urban areas, are more vulnerable to 

contamination from chemicals associated with urban activities.

Routine testing of water from domestic wells is not required and homeowners are responsible for the testing, maintenance, or treatment of the water from their domestic well. Water that is provided to consumers from public-supply wells is 

required to be tested on a routine basis to help assure that the water meets Federal and State water-quality standards.

Finally, some groundwater samples were collected from monitoring wells. Monitoring wells, which typically do not have 

permanent pumps installed, are used for measuring water levels or occasionally collecting water samples with portable pumps, 

but are not used for drinking water, irrigation, or other purposes. Monitoring wells were installed by NAWQA to sample groundwater near the water table, so monitoring wells are shallower than domestic or public-supply wells located in the same areas.

For more information on the quality of groundwater from domestic wells sampled in many areas of the Nation, please see 

DeSimone and others.(2) For more information on water from public-supply wells sampled in many areas of the Nation, please 

see Toccalino and Hopple.(3)

Chapter 2

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 9
Anatomy of a well

A well is really just a hole in the ground (well bore or borehole) 

from which water can be removed. The well bore is lined with 

a well casing, such as a pipe, to prevent the well bore from 

collapsing. The casing, along with a sealant (called grout), also 

prevents water from flowing into the well from the land surface 

or from parts of the aquifer where the water quality may be less 

desirable. The casing can be open at the bottom or perforated 

at a specific depth with a screen, allowing water to flow into 

the well where it can be pumped to the surface. Coarse sand or 

gravel (called sand pack or gravel pack) can be placed around 

the well screen to help improve flow of water into the well. Some 

wells are cased only near the land surface, allowing water to 

flow into the well from nearly the entire length of the well bore.

When pumped, groundwater flows through the 

screened interval and 

up and out of the well

Unsaturated

Saturated

Water table

Screen

Well bore

Water level

in well

Wellhead

Well casing

Pump

Grout

Gravel pack

Massive central core—

confining layer

Rubble zone—

unconfined aquifer

Rubble zone—

confined aquifer

Unsaturated zone Water table

Wells

Surface water

Aquifer (aq.ui.fer)—äkwefer

An underground layer of saturated permeable materials (rock, gravel, sand, or silt) that will yield 

a useful quantity of water to a well.

Sedimentary aquifer

Groundwater storage

and flow between grains

of sediment

Bedrock aquifer

Groundwater storage

and flow in fractures

Screen

An unconfined aquifer is bounded at its top by the 

water table, below which water fills all the pore spaces 

in the rock. Water from the land surface can move 

down into an unconfined aquifer.

A confining layer is a layer of material (often clay) 

through which water does not easily flow, creating a 

boundary between aquifers. 

A confined aquifer is bounded at its top by a confining 

layer. Water enters or “recharges” confined aquifers 

where the confining layer is not present. Where the 

confining layer is not continuous or is breached (for 

example, by a well), flow between the unconfined and 

confined aquifer can occur. 

The pressure within a confined aquifer can be greater 

than that in the overlying unconfined aquifer if the 

source of the water in the confined aquifer is at a higher 

elevation than the unconfined aquifer. In that case, 

water in a well in a confined aquifer will rise to a higher 

level than that in the overlying unconfined aquifer. 

The unsaturated zone is the area below the land 

surface and above an aquifer. In addition to soil, 

rocks, and air, it contains water from the land surface 

(such as rain) that is slowly moving downward to the 

water table of the aquifer. 

10 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 2

Assessing Water Quality in the Columbia 

Plateau, Snake River Plain, and Oahu Aquifers

To provide a broad assessment of groundwater-quality 

conditions in areas with mixed land uses across a large 

geographic area, one sample was collected from each of 

285 randomly selected,(4) existing domestic and publicsupply wells located in the three study areas (figs. 2–2, 

2–3, 2– 4; table 2–1). Samples were analyzed for major 

and trace inorganic constituents, nutrients, pesticides, and 

volatile organic compounds (VOC) (see sidebar, NAWQA 

assessments use a wide range of geochemical data and site 

information, p. 14). Samples were collected from the basalt 

aquifers of the Columbia Plateau, the Snake River Plain, and 

Oahu (appendix 1) and from major alluvial aquifers of the 

Snake River Plain. In the Columbia Plateau and on Oahu, 

most samples were collected from public-supply wells, so 

the results provide valuable insight into the quality of public 

drinking-water resources. In the Snake River Plain, most 

samples were collected from domestic wells, so the results 

provide insight on the quality of private drinking-water 

resources. To assess whether groundwater quality is changing 

in areas of mixed land use in the Columbia Plateau, 43 wells 

were resampled about 10 years later (table A1–1). 

To assess the quality of groundwater underlying 

agricultural land, one sample was collected from each of 

233 randomly selected wells in the Columbia Plateau and 

Snake River Plain (table 2–1). Groundwater studies in 

agricultural areas were designed to evaluate how a single crop 

type, irrigation regime, or other key characteristic affects water 

quality (table A1–1). In the Quincy-Pasco, Washington, area of 

the Columbia Plateau, samples of groundwater underlying two 

types of agricultural land—irrigated row crops and orchards—

were collected. Further, for each of the two land-use types, 

groundwater was collected at two depths: Shallow groundwater 

near the water table was sampled from monitoring wells that 

withdraw groundwater from the basin-fill sediments, and 

deeper groundwater was sampled from domestic wells that 

withdraw groundwater from a mixture of basin-fill sediments 

and basalt rock (fig. 2–2). In the Palouse, Washington, area 

of the Columbia Plateau, groundwater underlying dryland 

grains was sampled. Two types of wells also were sampled 

in the Palouse area—shallow monitoring wells that withdraw 

groundwater in wind-blown silt (loess) sediments, and 

domestic wells that withdraw deeper groundwater from 

the underlying basalt aquifer. In the Snake River Plain, 

samples of groundwater underlying irrigated row crops were 

collected in four areas. In three of those areas, the wells 

tap the basalt aquifer, and in one area, the wells tap a local 

sedimentary aquifer overlying the basalt aquifer (fig. 2–3). 

Monitoring wells in the Quincy-Pasco and Palouse areas of 

the Columbia Plateau, and domestic wells in the A&B, Idaho, 

and Jerome/Gooding, Idaho, areas of the Snake River Plain 

(fig. A1–2; table A1–1), were resampled about 10 years later 

to assess whether the groundwater quality had changed. No 

studies of groundwater quality underlying agricultural lands 

were conducted on Oahu.

In the Columbia Plateau, a special study was conducted 

to investigate the occurrence of soil fumigants in an area 

with a history of fumigant contamination (table A1–1). For 

this study, samples collected from a network of domestic 

wells were analyzed using laboratory methods that can 

detect 1,2-dibromoethane (ethylene dibromide, EDB) and 

1,2-dibromo-3-chloropropane (DBCP) at concentrations 

much lower than those typically reported. On Oahu, a special 

study involved the use of monitoring wells to evaluate 

contamination in a zone of groundwater near the water table 

(called the irrigation-recharge layer). The irrigation-recharge 

layer overlies the groundwater sampled from the publicsupply wells on Oahu.

Table 2–1. Summary of NAWQA groundwater studies in the Columbia Plateau and Snake River Plain and on Oahu discussed in this 

report. Appendix 1 describes the groundwater studies in greater detail.

[—, not studied] 

Groundwater

study area

Number of wells sampled (and median well depth) in groundwater studies

Agricultural lands Mixed land uses

Groundwater near

water table

tapped by

monitoring wells

Deeper

groundwater

tapped by 

domestic wells

Groundwater near

water table

tapped by

monitoring wells

Deeper

groundwater

tapped by 

domestic wells

Deeper

groundwater

tapped by publicsupply wells

Columbia Plateau 51 (36 feet) 77 (134 feet) — — 139 (185 feet)

Snake River Plain — 105 (190 feet) — 101 (182 feet) —

Oahu — — 15 (693 feet) — 30 (512 feet)

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 11
EXPLANATION

Regional study area of mixed land uses

Palouse area, agricultural lands

Quincy-Pasco area, agricultural lands

Well sampling groundwater underlying

 Agricultural lands, domestic

 Agricultural lands, monitoring

 Mixed land uses, public supply

0 25 50 MILES

0 25 50 KILOMETERS

FIGURE 2-2.

Palouse

Richland

Kennewick

Coulee City

N

OREGON IDAHO

WASHINGTON

Map

area

Jackson

Pocatello

Sun Valley

0 25 50 75 100 MILES

0 25 50 75 100 KILOMETERS

FIGURE 2-3

EXPLANATION

Regional study area of mixed land uses

Agricultural lands study area

Well sampling groundwater in

Basalt aquifer underlying agricultural

 lands, domestic

Sedimentary aquifer underlying agricultural

 lands, domestic

Basalt and sedimentary aquifers underlying

 mixed land uses, domestic and public supply

N

IDAHO

MONTANA WYOMING

NEVADA UTAH

Map area

Figure 2–3. In the Snake River Plain, 105 wells in agricultural lands and 101 wells in areas of mixed land uses were 

sampled to evaluate groundwater quality across the study area. Appendix 1 describes the Snake River Plain study 

area in greater detail.

Figure 2–2. In the Columbia Plateau, 267 wells were sampled to assess groundwater quality in a variety of land-use 

settings—51 monitoring wells in agricultural lands, 77 domestic wells in agricultural lands, and 139 public-supply wells 

in areas of mixed land uses. Appendix 1 describes the Columbia Plateau study area in greater detail.

12 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 2

Wahiawa

Waialua

Honolulu

N

HAWAII

Map area

EXPLANATION

FIGURE 2-4

Regional study area of mixed land uses

Well sampling groundwater underlying

 Mixed land uses, monitoring

 Mixed land uses, public supply

0 4 6 8 MILES 2

0 4 6 8 KILOMETERS 2

Figure 2–4. Fifteen monitoring wells and 30 public-supply wells were sampled on Oahu to evaluate groundwater 

quality underlying areas of mixed land uses. Appendix 1 describes the Oahu study area in greater detail.

Sugar beets are an important row crop in the Snake 

River Plain. NAWQA studies in the Snake River Plain 

were designed to evaluate the effects of irrigated 

row-crop agriculture on groundwater quality.

This air-curtain orchard sprayer uses multiple cross flow fans 

to disperse pesticide to apple trees. Under some conditions, 

the smooth, gentle flow of air can substantially reduce spray 

drift. Groundwater underlying orchards was sampled in the 

Quincy-Pasco, Washington, area of the Columbia Plateau to 

investigate the occurrence of nitrate and pesticides.

Photograph by Scott Bauer, USDA ARS

Photograph by Keith Weller, USDA ARS

Chapter 2: NAWQA Approach to Assessing Groundwater Quality 13
Chemists at the USGS National Water Quality Laboratory analyze 

groundwater samples using an array of sophisticated techniques.

Additional information (often called “ancillary information”) about 

the well and surrounding environment complements the chemical 

data measured. This additional information often is key to making 

sense of the chemical data. For example, the information might 

be used to determine that shallow groundwater is more (or less) 

vulnerable to contamination than deep groundwater, that domestic 

wells are more (or less) vulnerable to contamination than publicsupply wells, or that urban land use is associated with different 

types of groundwater contamination than is agricultural land use. 

Chemical data without accompanying ancillary data are much less 

useful for understanding factors that affect groundwater quality. 

Constituents measured in samples from most wells

Constituent group Examples

Water-quality properties pH, specific conductance, dissolved 

oxygen, temperature

Major ions (filtered) Bromide, calcium, chloride, 

magnesium, sodium, sulfate

Trace elements (filtered) Arsenic, boron, iron, manganese, 

selenium, uranium

Nutrients (filtered) Ammonia, nitrate, phosphorus

Pesticides (filtered) Herbicides, insecticides, fungicides

Volatile organic 

compounds

Solvents, gasoline hydrocarbons, 

refrigerants, trihalomethanes, 

fumigants

Organic carbon (filtered)

Additional constituents measured in samples from some wells

Constituent group Examples

Radionuclides Radon

Groundwater age tracers Tritium, chlorofluorocarbons

Stable isotopes Oxygen-18, hydrogen-2

Microorganisms Escherichia coli and total coliforms

Additional site information

Use of well

Well depth

Depth to water

Well-construction data

Principal Aquifer

Land-surface elevation at well

Land use within a 500-meter 

(1,640-foot) radius buffer

Estimates of nutrient inputs

Estimates of pesticide use

NAWQA assessments use a wide range of geochemical data and site information

14 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
A, Irrigation has allowed the Columbia 

Plateau to become some of the most 

productive agricultural areas in the 

United States. B, Pineapple cultivation 

on Oahu uses drip irrigation to reduce 

the harmful effects of root damage from 

nematodes and to reduce moisture 

stress to the plants during dry periods. 

C, The popular Idaho potato could 

not be grown in the Snake River Plain 

without extensive irrigation systems.

Chapter 3

Photographs by A, Doug Wilson, USDA ARS; B, Peggy Greb, USDA ARS; C, ©1976 by W.H. Mullins and published with permission

There are striking similarities—and differences—among the Columbia 

Plateau, Snake River Plain, and Oahu aquifers. All three areas contain 

extensive regional aquifers of a hard, gray, volcanic rock called basalt, 

which sets these three Principal Aquifers apart from all others in the United 

States. Although the aquifer materials are similar among the three areas, 

differences in agricultural practices and climate affect the hydrogeologic 

systems and groundwater quality of the areas. Much of the land in the 

Columbia Plateau and Snake River Plain is arid desert; without irrigation, 

agriculture would not be possible. In contrast, the tropical climate on Oahu 

allows crops such as pineapple and sugarcane to be grown. The differences 

between irrigation methods and types of crops grown in the three areas affect 

the amount of groundwater recharge and the types of chemicals transported 

to the groundwater along with the recharge.

A B

C

Chapter 3: Environmental and 

Hydrogeologic Setting 

This chapter summarizes

background information

for the Columbia Plateau,

Snake River Plain, and 

Oahu aquifers, and 

provides the context for 

understanding findings 

about water quality in 

these Principal Aquifers. 

The chapter covers 

the environmental and 

hydrogeologic setting, 

including population, 

land use, and water use.
Main Hawaiian Islands

Honolulu

Wahiawa

Haleiwa

Kauai

Niihau

Oahu

Oahu

Molokai

Maui

Hawaii

Lanai

Kahoolawe

IDAHO

OREGON

MONTANA

NEVADA UTAH WYOMING

124° 120° 116° 112°

48°

46°

44°

42°

0 100 150 200 MILES 50

0 100 150 200 KILOMETERS 50

CALIFORNIA

0 50 75 100 MILES 25

0 50 100 KILOMETERS

0 6 MILES3

0 6 KILOMETERS 3

160° 159° 158° 157° 156° 155°

22°

21°

20°

19°

WASHINGTON

EXPLANATION

Columbia Plateau basin-fill aquifers

Columbia Plateau basaltic-rock aquifers

Northern Rocky Mountains Intermontane Basins 

 aquifer system

Pacific Northwest basin-fill aquifers

Snake River Plain basin-fill aquifers

Snake River Plain basaltic-rock aquifers

NAWQA Central Columbia Plateau study area

NAWQA Upper Snake River Basin study area

EXPLANATION

Hawaiian volcanic-rock aquifers 

(overlain by sediments in places)

NAWQA Oahu study area

Figure 3-1.

A B

Environmental Setting

The Columbia Plateau, the Snake River Plain, and the 

Hawaiian Islands are large volcanic areas in the western 

United States and mid-Pacific Ocean, respectively (fig. 3–1). 

Each area contains extensive regional aquifers of a hard, 

gray, volcanic rock called basalt. The Columbia Plateau 

aquifers encompass 50,600 square miles in parts of Washington, Oregon, and Idaho. The Snake River Plain aquifers 

encompass 15,600 square miles across southern Idaho and a 

small sliver of eastern Oregon. The aquifers underlying the 

Hawaiian Islands encompass only 6,446 square miles. For 

additional details about the environmental setting, geology 

and groundwater hydrology in these three areas, refer to the 

USGS Ground Water Atlas of the United States.(5)

Two Principal Aquifers in the Columbia Plateau drainage 

basin are included in this assessment: the Columbia Plateau 

basin-fill aquifers and the Columbia Plateau basaltic-rock 

aquifers (fig. 3–1; appendix 1).(5) The basaltic-rock aquifers 

are overlain by the basin-fill aquifers, which are composed 

of unconsolidated and semiconsolidated sand and gravel. For 

brevity, hereafter in this report, this study area will be referred 

to as the Columbia Plateau.

In the Snake River Plain, the primary focus of study 

was the Snake River Plain basaltic-rock aquifers in the 

Upper Snake River Basin, which encompasses the Eastern 

Snake River Plain and surrounding tributary valleys 

(fig. 3–1; appendix 1). Four Principal Aquifers are located 

in the Upper Snake River Basin: the Northern Rocky 

Mountains Intermontane Basins aquifer system, the Pacific 

Northwest basin-fill aquifers, the Snake River Plain basin-fill 

aquifers, and the Snake River Plain basaltic-rock aquifers.(5)

Groundwater samples were collected from all four Principal 

Aquifers, but most samples were collected from the Snake 

River Plain basaltic-rock aquifers. For brevity, hereafter in this 

report, the Upper Snake River Basin will be referred to as the 

Snake River Plain study area.

In Hawaii, groundwater quality was assessed on the 

island of Oahu (fig. 3–1). The most important aquifers are 

composed of volcanic rocks. Consolidated sedimentary 

deposits, which are principally coralline limestone, form 

productive aquifers in lowlands and nearshore areas, 

but generally contain brackish water or saltwater.(5) The 

sedimentary aquifers were not sampled because they are 

not important drinking-water resources. 

Figure 3 –1. A, The Columbia Plateau and Snake 

River Plain in the western United States are underlain 

by extensive aquifers of sediments and basalt. 

B, The Hawaiian Islands are underlain by volcanicrock aquifers (mostly basalt). Groundwater was 

sampled only on the island of Oahu (inset).

16 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 3

i1359h3blbkd

The Columbia Plateau, 

 Snake River Plain, and

 Oahu aquifers at a glance.....

Percent increase in population since 1970

The number of people living in the area in 2005

Mean annual precipitation

Groundwater used for irrigation

Groundwater used for public supply

Groundwater used by domestic wells

Average thickness of individual basalt flows

The most commonly detected contaminant above the USEPA MCL

Percent of wells with nitrate concentrations above the USEPA MCL

160

821,000

Less than 15 inches

810 million gallons per day

223 million gallons per day

33 million gallons per day

30 to 300 feet thick

Nitrate

20

225

491,000

Less than 15 inches

2,900 million gallons per day

150 million gallons per day

34 million gallons per day

20 to 100 feet thick

Nitrate

4 

Percent increase in population since 1970

The number of people living in the area in 2005

Mean annual precipitation

Groundwater used for irrigation

Groundwater used for public supply

Groundwater used by domestic wells

Average thickness of individual basalt flows

The most commonly detected contaminant above the USEPA MCL

Percent of wells with nitrate concentrations above the USEPA MCL

140

902,000

70 inches

171 million gallons per day

243 million gallons per day

12 million gallons per day

3 to 30 feet thick

Soil fumigants

 

0

Percent increase in population since 1970

The number of people living in the area in 2005

Mean annual precipitation

Groundwater used for irrigation

Groundwater used for public supply

Groundwater used by domestic wells

Average thickness of individual basalt flows

The most commonly detected contaminants above the Hawaii 

 or USEPA MCLs

Percent of wells with nitrate concentrations above the USEPA MCL 

The Columbia Plateau

The Snake River Plain

Oahu, Hawaii

Photograph by USDA ARS Photograph by Michael G. Rupert, USGS

Photograph copyright Kestrel Aerial Services, Inc., ©www.kestrelaerial.com, used with permission

Chapter 3: Environmental and Hydrogeologic Setting 17
1,000,000

900,000

800,000

700,000

600,000

500,000

400,000

300,000

200,000

Study area population

Columbia Plateau

Snake River Plain

Oahu

1970 1975 1980 1985 1990 1995 2000 2005

Year

EXPLANATION

Population

Figure 3-2.

Modified from U.S. Census Bureau(83)

Figure 3–2. The number of people living in the 

Columbia Plateau and Snake River Plain and 

on Oahu has increased steadily since 1970. As 

population grows, so does the demand for goodquality drinking water. 

18 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005

There is a large difference in climate among the 

three study areas, which, in turn, affects the amount of 

groundwater recharge and the movement of contaminants 

to the groundwater. The temperature range in the Columbia 

Plateau and Snake River Plain is extreme—low temperatures 

during the winter are below freezing and high temperatures 

during the summer exceed 100 degrees Fahrenheit (°F). 

Climate in the lower elevations of the Columbia Plateau and 

Snake River Plain is dry, with mean annual precipitation 

less than 15 inches over large areas and less than 8 inches 

in some places. Because the climate is so dry, most crops 

must be irrigated, although dryland (non-irrigated) crops 

such as winter and spring wheat can be grown in some of 

the wetter areas. Very little groundwater recharge occurs in 

the non-irrigated areas, but recharge from excess irrigation 

water can occur in irrigated areas, potentially transporting 

nitrite plus nitrate as nitrogen (nitrate) and pesticides with it. 

Precipitation in adjacent mountains and tributary valleys can 

be 45 to 60 inches or more;(6, 7) runoff from these mountainous 

regions provides surface-water irrigation waters and groundwater recharge to lower elevation areas.

In contrast, Oahu has a subtropical climate. Temperatures 

are mild and vary little. August is the warmest month of the 

year, with a mean temperature of 80.5 °F, and the coolest 

month is February, with a mean temperature of 72 °F.(8)

Precipitation on Oahu varies greatly with elevation and 

exposure to prevailing easterly trade winds—from less than 

25 inches per year near Honolulu to more than 275 inches 

in the surrounding mountains, one of the wettest places in 

the United States.(9) Rainfall varies dramatically over short 

horizontal distances; in some places, this variation can be as 

much as 80 inches over a horizontal distance of just 1 mile.

The climate can affect the types of contaminants that 

leach to the groundwater. For example, the climates in the 

Columbia Plateau and Snake River Plain are similar, and 

similar row crops are grown in both areas, so the types of 

pesticides used are similar (with the exception of pesticides 

used on specialty crops such as orchards in the Columbia 

Plateau or sugar beets in the Snake River Plain). The climate 

on Oahu and the crops grown (such as pineapple) are different 

from the Columbia Plateau or the Snake River Plain, so many 

of the pesticides used are different. The pesticide application 

rates are sometimes high on Oahu because of the difficulty in 

controlling insects and weeds in a tropical climate.

Population

A growing population increases the demand for goodquality drinking water. Population is increasing in all three 

areas—from 1970 to 2005, the population increased by 

160 percent in the Columbia Plateau, by 225 percent in the 

Snake River Plain, and by 140 percent on Oahu (fig. 3–2). 

The population in the Columbia Plateau (821,000 in 2005) 

is widely distributed across rural farming communities and 

urban centers. The most densely populated urban area is also 

the most densely cultivated area—the southwest part of the 

Columbia Plateau near the tri-city area of Pasco-RichlandKennewick (fig. 3–3). Because of the dense population, this 

area has the greatest nitrogen input and pesticide use. Similar 

to the Columbia Plateau, the population in the Snake River 

Plain (491,000 in 2005) is distributed among rural farming 

communities and urban centers such as Twin Falls, Pocatello, 

and Idaho Falls (fig. 3–4). Most of the urban communities are 

clustered near the Snake River or along its flood plain, and 

rural communities are scattered in tributary valleys and farther 

from the Snake River. The urban communities are adjacent 

to the most productive agricultural lands, so groundwater 

resources that provide drinking water for urban residents may 

be affected by agricultural activities on the surrounding lands. 

Although more people live in the Columbia Plateau than in the 

Snake River Plain, the population density is roughly the same. 

Oahu is the most populous (902,000 in 2005), most densely 

populated, and most urbanized of the three areas. Most of 

the population is concentrated in urban Honolulu and nearby 

suburban communities on southern Oahu (fig. 3–5). The 

population density on Oahu is about 100 times greater than 

that in the Columbia Plateau and Snake River Plain. 
Chapter 3

Pasco

Moscow

Hooper

Quincy

Pullman

Palouse

Othello

Spokane

Lewiston

Richland

Kennewick

Moses Lake Ritzville

Coulee City

Grand Coulee

Coeur d'Alene

120° 119° 118° 117°

48°

47°

OREGON IDAHO

WASHINGTON

Columbia

Plateau

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

Base modified from U.S. Geological Survey 1:2,000,000-scale digital data (2005), NAD 83 datum

Albers Equal Area Conic projection, standard parallels 29°30'N and 45°30'N, central meridian 119°W

Land use/land cover is circa 2001 from the National Land Cover Database(84)

EXPLANATION

 Agriculture

 Grassland

 Rangeland (shrub/scrub)

 Forest

 

 Columbia Plateau study area

Figure 3-3A.

 Barren/transitional

 Urban

 Open water

 Wetland

Land use/land cover

A

Figure 3–3. Land use is predominantly agricultural (yellow) in the Columbia Plateau. 

Chapter 3: Environmental and Hydrogeologic Setting 19
Buhl Eden

Arco

Teton

Burley

Jerome

Gooding

Jackson

Hagerman Pocatello

King

Hill

Sun Valley

Twin Falls

Idaho

Falls

American Falls

114° 112° 110°

44°

43°

42°

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

Eastern Snake River Plain

Snake River Basin study area

B

Figure 3-3B.

Base modified from U.S. Geological Survey 1:2,000,000-scale digital data (2005), NAD 83 datum

Albers Equal Area Conic projection, standard parallels 29°30'N and 45°30'N, central meridian 113°W

Land use/land cover is circa 2001 from the National Land Cover Database(84)

EXPLANATION

 Agriculture

 Grassland

 Rangeland (shrub/scrub)

 Forest

 

 Barren/transitional

 Urban

 Open water

 Wetland

Land use/land cover

IDAHO

MONTANA WYOMING

NEVADA UTAH

Snake

River Plain

Snake

River

Thousand Springs

Figure 3–4. Agricultural land use is extensive in the central part of the Snake River Plain and the tributary valleys south and east 

of the plain. Rangeland and forested lands also make up a large percentage of the land use in the Snake River Plain. Agricultural 

land use has the greatest potential to adversely affect groundwater quality. 

20 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 3

Ewa

Aiea

Kailua

Waipio

Makaha

Kahuku

Kaneohe

Waipahu

Wahiawa

Waianae

Waialua

Honolulu

Kunia

Camp

Sunset

Beach

Mililani

Town

Base modified from U.S. Geological Survey 1:24,000-scale digital data (2003), NAD 83 datum

Albers Equal Area Conic projection, standard parallels 8°N and 18°N, central meridian 157°W

Land use/land cover is circa 1998 from Klasner and Mikami(85)

158°10' 158° 157°50' 157°40'

21°40'

21°30'

21°20'

0 3 6 9 12 MILES

0 3 6 9 12 KILOMETERS

 Agriculture

 Urban

 Urban, converted from agricultural use from 1950 to 1998

 Other (mostly forest)

Figure 3-3C.

NAWQA Oahu study area

EXPLANATION

Land use/land cover

Figure 3–5. Agricultural land use extends over much of central Oahu, but many historically agricultural lands have been 

converted to urban land use.

HAWAII

Map area

Chapter 3: Environmental and Hydrogeologic Setting 21
0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

Withdrawals, in millions of gallons per day

Columbia Plateau 

Snake River Plain 

Oahu

EXPLANATION

Groundwater withdrawals

Figure 3-4.

Surfacewater

irrigation

Groundwater

irrigation

Public

supply

Selfsupplied

industrial

Domestic

Land Use and Water Use

Land use in the three study areas is diverse (figs. 3–3 to 

3–5). Principal crops in the Columbia Plateau and Snake River 

Plain are row crops such as alfalfa, cereal grains (barley, oats, 

and wheat), potatoes, and corn. The Columbia Plateau also 

has orchards (primarily apples and cherries) and vineyards. 

Sugar beets are an important crop in the Snake River Plain. 

Agriculture on Oahu was dominated historically by sugarcane 

and pineapple, but sugarcane was phased out in 1996, leaving 

pineapple, orchards, and vegetables. The total number of 

cultivated acres has remained fairly constant in the Columbia 

Plateau and Snake River Plain, but has decreased on Oahu 

in recent decades, as former agricultural lands in central and 

southwest Oahu have been converted to suburban and light 

commercial use since the 1960s.

Of the various land uses in the three study areas, urban 

and agricultural land use have the greatest potential to 

adversely affect groundwater quality. The quality of groundwater underlying urban land can be affected by chemical 

leaks and spills, leachate from landfills, pesticide and fertilizer 

use, and wastewater disposal. The quality of groundwater 

underlying agricultural land can be affected by leaching of 

fertilizer, manure, and pesticides applied at the land surface. 

Crop rotation, tillage practices, and irrigation practices can 

affect groundwater quality by either promoting or reducing 

the leaching of contaminants to the groundwater.

The high crop productivity in the Columbia Plateau and 

Snake River Plain is possible only through irrigation. Crop 

irrigation is the dominant use of surface and groundwater in 

these areas (fig. 3–6). Some of this irrigation water is applied 

to fields in excess of what is needed by plants, so the excess 

Figure 3–6. Crop irrigation is the dominant use of 

groundwater in the Columbia Plateau and Snake River

Plain, but public supply is the dominant use of groundwater on Oahu. Maintaining good groundwater quality 

is essential to providing water for drinking, for livestock, 

and for the agricultural food-processing industry.

water infiltrates down to the water table, potentially carrying 

nitrate and pesticides with it. Although domestic wells provide 

drinking water for a substantial part of the population in the 

Columbia Plateau and Snake River Plain, the actual amount of 

water pumped by a domestic well is small, so the overall water 

use by domestic wells is small. On Oahu, public water supply 

is the largest use of groundwater, but groundwater use for 

irrigation also is substantial.

The Snake River Plain basalt-rock aquifer supplies large volumes 

of groundwater from great depths for irrigation. Here, a single 

well is producing over 1,000 gallons of groundwater each minute 

to irrigate the surrounding fields. There are over 100 of these 

irrigation wells in the A&B area of the Snake River Plain (fig. A1–2), 

where groundwater is pumped and applied to crops, and then 

excess irrigation water is recharged back to the groundwater. This 

cycle causes dissolved constituents, such as salts and nitrate, to 

become more concentrated. 

Extensive irrigation canal systems divert water from the Snake 

River and distribute it across the Snake River Plain to irrigate 

fields. The Northside Canal, shown here, is located near Milner 

dam, where the entire flow from the Snake River is diverted for 

irrigation. Recharge from irrigation water has the potential to 

carry nitrate and pesticides to the groundwater in the Snake 

River Plain aquifers. 

Photograph by Charles D. Hunt, Jr., USGS

Photograph by Michael G. Rupert, USGS

22 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 3

Rubble zone (flow-top 

breccia, aa clinker), 

high permeability

Massive central core, 

minimal permeability

Rubble zone (flow-top 

breccia, aa clinker), 

high permeability

Base of overlying flow (not shown)

Modified from Whitehead(86)

Figure 3-5.

Hydrogeologic Setting

The largest and most productive aquifers in the Columbia 

Plateau and the Snake River Plain are composed of basalt, 

and each area is overlain by smaller sedimentary aquifers. On 

Oahu, there is one primary basaltic-rock aquifer. Movement 

of groundwater is primarily through fractures, voids, and 

conduits in the basaltic rocks and through intergranular pores 

in the sediments, where present.

The physical characteristics of the basalt aquifers allow 

groundwater and contaminants to flow quickly through the 

aquifers. Individual basalt flows have three zones: (1) a top 

layer of rock fragments (known as flow-top breccia in the 

Columbia Plateau and Snake River Plain and as rubble or 

“aa clinker” on Oahu), (2) a hard, dense, massive core, and 

(3) a basal layer of rock fragments (rubble) (fig. 3–7). The 

top and bottom layers are composed of rubble because the 

outer parts of the lava flows cooled quickly and crumbled 

apart; the middle layer is hard and dense because it cooled 

more slowly. Most of the groundwater and contaminants 

flow through the rubble zones. Much less groundwater 

flows through the massive central core because it is much 

less fractured; in the Columbia Plateau these middle 

layers are so thick and dense that they can act as confining 

Figure 3–7. A typical basalt flow contains layers of different permeability: a dense, massive central 

core overlain and underlain with rock fragments known as rubble or “aa clinker” in Hawaii and as 

flow-top breccia in Washington and Idaho. These rubble zones are very permeable, which allows 

large amounts of groundwater and contaminants to flow rapidly through them. 

layers, inhibiting groundwater and contaminant movement. 

Contaminants that reach the water table in basalt aquifers can 

migrate great distances because the rubble zones can allow 

large amounts groundwater to flow rapidly. Basalt does not 

react with most contaminants, such as nitrate or pesticides, so 

contaminants can persist for long periods and be transported 

for long distances.

Although the Columbia Plateau, Snake River Plain, and 

Oahu share a common rock type (basalt), the thickness and 

geometry of the basalt layers differ because of the environments in which the rocks were deposited and the ways in which 

the basalts erupted from the volcanoes. These differences, in 

turn, affect groundwater flow and the movement of contaminants. Columbia River and Snake River basalts erupted within 

large subsiding basins that contained major rivers and lakes. 

As a result, these aquifers contain sequences of basaltic rocks 

(from the volcanic eruptions) interlayered with sedimentary 

deposits (from the rivers and lakes). The sedimentary deposits 

are less permeable than the rubble zones in the basalt, so they 

limit the vertical movement of groundwater and contaminants. 

Oahu basalts erupted on a dome-shaped island instead of in 

a large subsiding basin, and sedimentary deposits within the 

basalt flows are much less prevalent than in the Columbia 

Plateau or Snake River Plain.

Chapter 3: Environmental and Hydrogeologic Setting 23
Sequence of two individual basalt flows

Massive

central core

Sequence of

two individual

basalt flows

Massive

central core

Rubble

zone

Rubble

zone Sedimentary

interbed

In the Columbia Plateau, extremely high-volume eruptions produced individual lava sheets 30 to 300 feet thick that 

extend over several thousand square miles.(10) When all the lava 

sheets are considered together, the Columbia Plateau basalts 

are estimated to reach a maximum thickness of as much as 

15,000 feet.(5) The Columbia Plateau is considered a multiaquifer system, that is, a layered sequence of several basalt 

aquifers that are separated by layers of sediments, weathered 

rock, or massive basalt flow interiors (fig. 3–8).(11) The uppermost aquifer is susceptible to contamination from activities at 

the land surface, but the deeper aquifers are less susceptible 

because downward groundwater flow is limited by the layers 

of sediments and (or) the massive central cores of the basalt 

flows. Groundwater flows horizontally over great distances, as 

much as 100 miles. Sedimentary basin-fill aquifers are present 

where sediments have accumulated in low-lying basins to form 

complex, multilayered aquifer systems on top of the regional 

basalt aquifer.(12) Depth to the water table is 20 to 60 feet in the 

wells sampled for this study, but depth to the water table can 

be hundreds of feet in some places.

Eruptions of basaltic lavas in the Snake River Plain were 

smaller than in the Columbia Plateau. The Snake River lavas 

typically extend over 50 to 100 square miles and individual 

lava flows are 20 to 100 feet thick(13) with an estimated 

maximum total thickness of 5,500 feet. Because these flows 

are smaller in size, the massive central core of the lava flows 

is smaller than in the Columbia Plateau, and the rubble zones 

compose a larger percentage of the aquifer materials. Thus, 

the numerous lava flows behave like one aquifer, rather than 

several aquifers like the Columbia Plateau. As a result, groundwater and contaminants flow more readily throughout the 

Snake River Plain aquifer than through the Columbia Plateau 

aquifers. Although the depth to water is several hundred feet, 

there is little to obstruct movement of contaminants from the 

land surface down to the water table. Regional groundwater 

flow is to the southwest, where the groundwater discharges 

to the Snake River on the western edge of the study area in 

an area called Thousand Springs (fig. 3 – 4). The regionally 

extensive basaltic-rock aquifers are overlain by a thin layer of 

alluvial sediments and wind-blown loess, although bare lava 

is exposed in some areas of the Plain (fig. 3–9). A basin-fill 

aquifer was formed in the Minidoka area (appendix 1) when 

a basalt flow dammed the Snake River during the Pleistocene 

epoch, causing sediments to be deposited on top of the basalt 

aquifer.(14) Sediments that eroded from mountains near the 

margins of the Snake River Plain washed out onto the plain 

during its geologic history. As a result, sediments interfinger 

with basalt along the edges of the plain. Depth to the water 

table in the basalt is about 50 to 300 feet, and depth to the 

water table in sediments is 10 to 50 feet in most places.

Left: The thick, tabular Columbia River basalts are composed of numerous overlapping basalt flows from 30 to 300 feet thick. 

Right: The Snake River basalt flows are thinner, from 20 feet to more than 100 feet thick on average. The thick basalt flows in the 

Columbia Plateau can act as individual aquifers in some places, whereas the thinner flows of the Snake River Plain act as one 

interconnected aquifer.

Photographs by Charles D. Hunt, Jr., USGS

24 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 3

Rangeland Dryland wheat (no irrigation)

Basalt

NOT TO SCALE

Basin-fill

sediments

Basalt

Loess

Urban

Irrigated row crops

(alfalfa, potatoes,

wheat, others)

Orchards, vineyards

(apples, cherries,

grapes, others)

Quincy-Pasco area

Figure 3-6.

Palouse area

Regional groundwater flow

Water in fractured basalt

Multiple basaltic-rock aquifers

Basin-fill aquifer

Public-supply wells

Domestic well

Domestic well

Domestic well

Monitoring well

Monitoring well Monitoring well

Publicsupply

well

Domestic or

irrigation well

Domestic

well Irrigation well

Stock well

Perched water table in river 

and lake-bed sediments Rangeland

Pre-basalt

basement

rock

Sediments

and windblown loess

Basin-fill

sediments

Urban

Deep water

table in basalt

Dairy

Irrigated row crops

(potatoes, alfalfa, wheat,

sugar beets, others)

Basalt

Basalt

Fault associated with

basin subsidence

(arrows show

direction of slip)

Water in fractures

in the unsaturated zone

Shallow water table

Saturated

basalt

Unsaturated

basalt

Regional groundwater flow

Figure 3-7.

NOT TO SCALE

Figure 3–8. In the Columbia Plateau several basaltic-rock aquifers are present within large, 

individual basalt flows. Basin-fill alluvial sediments and loess (windblown silt) overlie the basaltrock aquifers throughout much of the Columbia Plateau. Groundwater from both the alluvial 

sediments and the basalt-rock aquifers is available for use.

Figure 3–9. The basalt-rock aquifer occupies most of the Snake River Plain. A thin layer of sediments 

and loess (windblown silt) covers the upper surface of the basalt. Where irrigation water is available, 

these sediments and loess provide productive farmland, but excess irrigation water can carry nitrate and 

pesticides to groundwater. Basin-fill sediments encroach from tributary valleys on the edges of the plain.

Chapter 3: Environmental and Hydrogeologic Setting 25
Water table Pacific

Sea level Ocean

NOT TO SCALE

Saltwater

Brackish

water

Freshwater

Figure 3-9.

Modified from Miller(5)

NOT TO SCALE

Forest

Saturated

Perched

Unsaturated

Unweathered

basalt

Weathered basalt

(saprolite)

Soil

Irrigation recharge 

layer (green)

Deep monitoring well extends as far as

1,500 feet below water table to monitor

freshwater-saltwater transition zone

Deep water table

Irrigated row crops

(pineapple, sugarcane, diversified crops;

sugarcane discontinued in 1996) 

Perched water table

Urban, military

Regional groundwater flow 

Urban

Publicsupply

well

Publicsupply

well

Publicsupply

well

Irrigation

well

Deep salinity

monitoring well

Monitoring

well

Monitoring

well

Perched water body

Unsaturated basalt

Deep water body

Figure 3-8.

Oahu lava flows are the thinnest and smallest among 

the three areas, averaging from just a few feet to 30 feet 

thick, and extending less than 50 square miles. Rubble 

zones on Oahu compose the largest percentage of the 

aquifer materials of the three study areas, so the Oahu 

aquifer is the most susceptible to contamination of the 

basalt aquifers in the three areas. The unsaturated zone on 

Oahu has a different composition than that of the Columbia 

Plateau and Snake River Plain (fig. 3–10). Because of 

the humid climate on Oahu, the basalt has decomposed, 

forming a saprolite, which is a red, soft, clay-rich deposit 

of weathered basalt. The coarse texture of the saprolite 

speeds the movement of contaminants from the surface 

down to the water table. Depths to groundwater range from 

100 to 800 feet. Freshwater extends 1,000 feet or more 

below the top of the water table, overlying saltwater from 

the surrounding ocean (fig. 3–11). The basalts extend to the 

ocean floor, about 16,000 feet below sea level.

Figure 3–10. The basalt-rock aquifer underlies the entire island of Oahu. Soil and weathered basalt 

(saprolite) are about 150 feet thick. Unsaturated, unweathered basalt extends several hundred feet 

below the saprolite to the deep water table. Small bodies of perched water are present on ash beds 

within the basalt flows. The existence of a distinct layer of “irrigation-induced recharge” containing 

pesticides and elevated nitrate has been known since the 1960s. Depth to the water table in central 

Oahu ranges from 100 to 800 feet in most places.

Figure 3–11. On oceanic islands such as Oahu, freshwater 

commonly develops as a freshwater lens that floats on saltwater 

and that is separated from the saltwater by a transition zone of 

brackish water. Drinking water and irrigation water are pumped 

from the freshwater lens, but wells can become contaminated 

with brackish water or saltwater if too much groundwater is 

pumped from the freshwater lens or if the island undergoes 

prolonged drought.

26 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 4

Photograph by Jack Dykinga, USDA ARS

Chapter 4: Natural Processes and Human Activities 

That Affect Groundwater Quality

This chapter explains 

and discusses the hydrologic and geochemical 

processes and human 

activities that affect the 

movement and quality 

of groundwater in the 

Columbia Plateau, 

Snake River Plain, and 

Oahu aquifers.

Aquifer hydrogeology and land-use practices affect the groundwater 

quality of the Columbia Plateau, Snake River Plain, and Oahu 

aquifers. For example, the natural characteristics of the materials 

overlying the Columbia Plateau, Snake River Plain, and Oahu aquifers allow 

water and chemicals to infiltrate to the water table even though the water table 

commonly is hundreds of feet below the land surface. Irrigation is intensive 

in all three study areas, which has caused leaching of fertilizers and pesticides 

applied at land surface down to the groundwater. The groundwater generally 

contains dissolved oxygen, which prevents nitrate and many pesticides from 

breaking down.

Most areas of the Snake River Plain receive 

less than 15 inches of rainfall each year, so 

irrigation is required to maintain high crop 

productivity. Excess irrigation water can 

infiltrate to the groundwater, transporting 

nitrate and pesticides with it.
Potentiometric surface

of Miocene basalticrock aquifers

Columbia

Columbia River

River

South North Modified from Lindholm and Vaccaro(87)

NOT TO SCALE

Figure 4-1.

EXPLANATION

Basin-fill aquifers

Miocene basaltic-rock aquifers

 Saddle Mountains Basalt

 Wanapum Basalt

 Grande Ronde Basalt

Confining unit

Aquifers in pre-Miocene rocks

Direction of groundwater movement

Vertical

fracture

Vertical

fracture

Small tubes

and voids

Rubble zone 

(flow-top breccia,

aa clinker), high

permeability

Massive central core, 

minimal permeability

The Geologic Characteristics of the 

Basalt Aquifers Make Them Susceptible 

to Contamination

The natural characteristics of basalt aquifers—welldrained soils, permeable volcanic rock, lack of regionally 

extensive clay layers—make them susceptible to contamination. Water moves from the land surface to the water table 

primarily through permeable unconsolidated sediments or 

along fractures and other “fast pathways” in the volcanic 

rocks. As a result, the water table in the Snake River Plain 

can rise rapidly (on the order of days or weeks) in response to 

large rainstorms or seasonal filling of irrigation canals, even 

though the water table may lie several hundred feet below 

Figure 4–1. In the Columbia Plateau aquifer system, groundwater flow is predominantly horizontal over distances of more 

than 100 miles, from recharge areas near the boundaries of the plateau eventually to the lower parts of the Columbia River. 

Groundwater moves through the confining units that separate the aquifers but at a slower rate than through the aquifers. 

the land surface.(15) The fractures and fast pathways allow 

contaminants to be carried along with infiltrating surface water 

down to the groundwater.

Once water infiltrates to the water table, it becomes part 

of the regional groundwater flow system. Groundwater flow 

is mostly horizontal over great distances, but can be vertically 

downward to deeper parts of the aquifers or upward to areas 

where groundwater discharges to streams or springs near the 

land surface (fig. 4–1). Contaminants in the groundwater can 

enter into drinking-water wells along the way. The time it 

takes groundwater to move through the entire regional flow 

system ranges from a few decades on Oahu to as much as 

350 years for flow through the entire Eastern Snake River 

Plain flow system.

Basalt aquifers typically have 

layers of a dense, massive central 

core that alternate with layers of 

rock fragments, known as rubble 

or “aa clinker” in Hawaii and as 

“flow-top breccia” in Washington 

and Idaho. The basalt flows 

can transmit large amounts of 

groundwater and contaminants 

because groundwater can flow 

rapidly through the rubble zones. 

The photograph shows thin-bedded 

basalt flows on Hawaii (Chain of 

Craters Road, Kilauea volcano); 

basalt flows in the Columbia Plateau 

and Snake River Plan are much 

thicker and larger than the basalt 

flows on the Hawaiian Islands.

Photograph by Clifford Voss, USGS

28 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 4

Certain Types of Irrigation of Agricultural Fields 

Can Impair Groundwater Quality

The high crop productivity in the Columbia Plateau 

and Snake River Plain and on Oahu is possible through the 

use of irrigation water diverted from rivers or pumped from 

underlying aquifers. When irrigation water is applied to fields, 

however, not all of it is used by plants. Some of the irrigation 

water can seep beyond the reach of plant roots and travel 

downward to the water table, carrying chemicals, such as 

fertilizers and pesticides, with it. This process is referred to 

in this report as “irrigation recharge.” When irrigation water 

is applied to fields, a portion of the water evaporates, leaving 

behind concentrations of dissolved minerals, nitrate, and salts, 

which further degrade the quality of the irrigation recharge. 

In some areas in the Snake River Plain, that groundwater is 

withdrawn again by wells and applied to fields, which sets 

up a cycle in which dissolved constituents such as salts and 

nitrate become more and more concentrated.(14)

The amount of irrigation water that infiltrates from the 

land surface depends on the irrigation method. Flooding 

fields with irrigation water is the least efficient method; on 

Oahu, about half of the water used in flood irrigation is lost to 

deep infiltration. Spray and drip irrigation are more efficient 

methods than flood irrigation. These more efficient irrigation 

methods can reduce the amount of contaminants that leach to 

the water table, because the total amount of irrigation recharge 

is reduced. Although most farmers have transitioned to more 

efficient irrigation methods, prior decades of flood/furrow 

irrigation contributed large volumes of irrigation recharge, 

which increased the amount and speed of groundwater flow 

in the three study areas.

On Oahu, fertilizers, pesticides, and soil fumigants 

applied at the land surface have leached from the soil and 

have been transported to the water table with the infiltrating 

irrigation recharge and precipitation, forming a distinct zone 

of groundwater about 50 to 200 feet thick near the top of 

the water table that has elevated concentrations of nitrate, 

pesticides, and other constituents (fig. 3–10).(16, 17) Publicsupply wells on Oahu typically have a solid, unscreened 

casing for the first 100 feet or so below the water table to 

prevent this poor-quality water from flowing into the wells. 

Left: Plowed furrows in the silty soils of the Snake River Plain are flooded from feeder ditches, and water flows down the furrows, wetting 

adjacent plant rows. Right: Most farmers in the Snake River Plain have converted to sprinkler irrigation systems, similar to that shown on the 

right, which are more efficient than furrow irrigation systems.

Photographs by Charles D. Hunt, Jr., USGS

Chapter 4: Natural Processes and Human Activities That Affect Groundwater Quality 29
What is groundwater age?

Groundwater age refers to the time elapsed since recharge 

water reached the groundwater table and became isolated from 

the atmosphere.(18, 19) The term “age” is sometimes qualified 

with the word “apparent” to signify that the accuracy of the 

determined age depends on many variables. Groundwater age 

can provide information on the susceptibility of the aquifer to 

contamination from chemicals related to human activities. Many 

chemicals related to human activities have no natural sources 

and were only developed during the last 60 years or so; these 

chemicals are unlikely to be detected in groundwater older than 

that. Estimates of groundwater age can be made by measuring a 

number of compounds and radioactive substances that infiltrate 

into the ground with the water and travel to the sampling 

point.(20, 21) Generally speaking, groundwater ages of less than 

60 years (recharged after 1952) are classified as “young” in the 

Principal Aquifers reported on in this series of circulars.

Massive central core—confining layer

Massive central core—confining layer

Rubble zone—

confined aquifer

Rubble zone—unconfined aquifer

Rubble zone—confined aquifer Centuries

Recharge area Discharge area Stream

Millennia

Pumping

well

Years

Days

NOT TO SCALE

Water table

Sidebar.

Columbia Plateau

Snake River Plain

Oahu

1930194019501960

1970

1980

19902000

2010

Estimated groundwater recharge date

Concentration, in milligrams per liter as nitrogen

30

25

20

15

10

5

0

Total annual nitrogen input from fertilizer 

in the United States, in millions of kilograms

12,000

10,000

8,000

6,000

4,000

2,000

0

Total nitrogen

fertilizer input

EXPLANATION

Nitrate concentration

Figure 4-2.

Young Groundwater Age Indicates a High 

Susceptibility to Contamination

Groundwater ages were determined for samples from 

a small number of wells in the Columbia Plateau and Snake 

River Plain and on Oahu. Most of the water sampled was 

recharged less than 60 years ago, indicating that groundwater in 

the sampled areas is susceptible to contamination from human 

activities because most contaminants associated with human 

activities came into use within the last 60 years. Groundwater 

age was determined for water from only 10 shallow wells in the 

Columbia Plateau, but the ages were quite young; the average 

age of shallow groundwater sampled from the sedimentary 

deposits in the Quincy-Pasco, Washington, area was only 

17.5 years.(22) Groundwater sampled from the Snake River Plain 

basalt aquifer was also quite young; the average groundwater 

age of water from 39 wells sampled was only 15 years.(23)

On Oahu, wells in two networks were sampled; one set of 

15 monitoring wells that withdraw groundwater from near the 

water table, and one set of 30 public-supply wells that withdraw 

groundwater much deeper below the water table. The average 

age of groundwater sampled near the water table was 20 years, 

and the average age of groundwater sampled much deeper 

below the water table was 32 years. The young groundwater 

ages at both depths indicates that the source of the groundwater 

is irrigation recharge that infiltrates to the water table from the 

cultivated fields and leaky irrigation canals.

Groundwater recharged after 1952 is more likely to 

contain contaminants related to human activities than older 

groundwater. For example, nitrate concentrations in groundwater in the three study areas tended to be higher in young 

groundwater (fig. 4–2). Before 1945, the primary sources of 

supplemental nitrogen (N) for crops were animal manure, 

mineral sources (such as potassium nitrate), and crop rotation 

with legume crops (such as alfalfa). After World War II (1945), 

facilities that had produced ammonia and synthetic nitrates 

for explosives were converted to the production of N-based 

fertilizers and fertilizer production rates increased dramatically. 

Nationally, the use of N fertilizer increased rapidly from 1950 

through about 1980 and has continued to increase at a slower 

rate since about 1980 (fig. 4–2). Nitrate concentrations in 

groundwater from the Columbia Plateau, the Snake River 

Plain, and Oahu reflect that increase in fertilizer use.

Figure 4–2. Nitrate concentrations are higher 

in groundwater recharged after 1952 because 

fertilizer use has increased in the United States 

since the 1950s.

30 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 4

Columbia Plateau

Palouse area, groundwater

near water table

Columbia Plateau

Quincy-Pasco area, row crops,

groundwater near water table

Columbia Plateau

Quincy-Pasco area, orchards,

groundwater near water table

Snake River Plain

Tributary valleys

Snake River Plain

Jackson valley

Snake River Plain

Minidoka area

Oahu groundwater

near water table

Oahu

public supply

Snake River Plain

A&B area

Snake River Plain

Snake River Plain

Columbia Plateau

public supply 

Snake River Plain

Eden area

Snake River Plain

Jerome/Gooding area

Columbia Plateau Quincy-Pasco area, 

orchards, domestic wells

Columbia Plateau

Quincy-Pasco area, 

row crops, domestic wells

Columbia Plateau

Palouse area, domestic wells

20

9

21

9

17

3

Sedimentary study areas

30 3

9

14

1

30

30 31 15 37

1

Basalt study areas

Mixed basalt and 

sedimentary study areas

3

6 19

1

21

1

28

1 1

13

5

3

2

13

The Amount of Oxygen in Groundwater Can Affect 

the Mobility and Persistence of Contaminants

The amount of dissolved oxygen in groundwater, which 

is the primary information used to classify the reductionoxidation or “redox state” of groundwater, is an indicator 

of the geochemical processes that can affect whether or not 

contaminants breakdown in groundwater.(24) The redox state 

of groundwater is classified as oxic if the groundwater contains 

at least 0.5 milligram per liter (mg/L) dissolved oxygen 

(fig. 4–3). Groundwater is classified as anoxic if it contains less 

than 0.5 mg/L of dissolved oxygen. Sometimes, both oxic and 

reduced groundwater can be drawn into the same well from 

different zones in the aquifer. If this happens, the groundwater 

is classified as mixed. If groundwater is oxic, nitrate is unlikely 

to break down and can persist in groundwater, sometimes for 

decades.(25) Under anoxic conditions, nitrate concentrations can 

decrease through a process called denitrification in which bacteria 

convert nitrate to inert nitrogen gas. Many pesticides, such as 

atrazine, can break down more quickly under anoxic conditions 

than under oxic conditions. In contrast, concentrations of other 

chemicals, such as petroleum hydrocarbons, can decrease more 

quickly under oxic conditions because the bacteria that consume 

petroleum hydrocarbons require water with dissolved oxygen.

Figure 4–3. Most groundwater samples collected in the Columbia Plateau, the Snake River 

Plain, and Oahu were found to represent oxic conditions. Oxic conditions were more common 

in groundwater from the basalt aquifers than from the sedimentary aquifers at least partly 

because greater amounts of organic material in the sedimentary aquifers lead to anoxic 

conditions. Some contaminants, such as nitrate and atrazine, can persist for decades in oxic 

groundwater, but other contaminants, such as hydrocarbons—components of gasoline—

degrade in oxic groundwater.

Redox state of aquifer—Number

 indicates number of samples

Oxic

Anoxic

EXPLANATION

19

6

2

Mixed

Figure 4-3. EXPLANATION

Chapter 4: Natural Processes and Human Activities That Affect Groundwater Quality 31
How do redox reactions work? 

Reduction/oxidation (redox) processes require one chemical species that donates electrons and another chemical species 

that accepts those electrons. As a chemical species donates electrons it is “oxidized,” and as the other species accepts 

electrons it is “reduced.” Redox processes typically are facilitated by microbes (bacteria), which use the energy produced by the 

processes. In groundwater, organic carbon is the most common electron donor. If dissolved oxygen is present, it is the preferred 

electron acceptor, because reduction of dissolved oxygen produces more energy than reduction of other chemical species that 

commonly occur in groundwater. The atmosphere is the source of the dissolved oxygen, so the redox conditions in an aquifer 

near where recharge occurs usually are oxic (defined here as having a concentration of dissolved oxygen of at least 0.5 mg/L).

As groundwater moves through the aquifer along a flow path, the dissolved oxygen in the groundwater gradually is 

consumed by redox processes. Once all of the dissolved oxygen is consumed, other chemical species can accept electrons and 

become reduced. If nitrate is present, it will become the preferred electron acceptor, until it in turn is completely consumed. 

This pattern continues, with manganese, iron, sulfate, and finally carbon dioxide acting as electron acceptors until they are 

consumed, in that order. This order of use of electron acceptors has important implications for the preservation, degradation, 

and even production of contaminants in groundwater. Because redox reactions occur in a sequence, it can take a long time for 

strongly reducing conditions to develop. For this reason, anoxic groundwater commonly is older than oxic groundwater, and, 

within the anoxic category, strongly reducing groundwater commonly is older than mildly reducing groundwater. 

From a water-quality perspective, denitrification—the reduction of nitrate to nitrogen gas—is one of the most important 

redox processes that occurs in groundwater. Nitrate is a concern for human health and, where it discharges to surface water, 

can impair aquatic communities. Conversion of nitrate by denitrification to harmless nitrogen gas, the same gas we breathe in 

the atmosphere, is the primary way that nitrate is removed from water.

Oxygen Nitrate Manganese Iron Sulfate Carbon dioxide

Oxic Anoxic

Oxic Mildly

reducing

Strongly

reducing

Very strongly

reducing

Order of consumption of electron acceptors

32 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 5

Chapter 5: Quality of the Groundwater Resource 

Used for Drinking 

This large public-supply 

well provides drinking 

water to several thousand 

residents in the Snake 

River Plain. These residents 

depend on a high quality, 

reliable source of drinking 

water. Other key aspects 

of the economy, such as 

livestock production and 

food processing, also 

require clean water.

Photograph by Kenneth Skinner, USGS

This chapter identifies 

and discusses constituents that were detected 

at a concentration 

greater than or near 

a human-health 

benchmark in water 

from drinking-water 

supply wells tapping 

the Columbia Plateau, 

Snake River Plain, and 

Oahu aquifers. 

Concentrations of at least one contaminant exceeded its human-health 

benchmark in 14 percent of drinking-water wells sampled in the 

Columbia Plateau, Snake River Plain, and Oahu aquifers. Some 

of these contaminants—radon, arsenic, manganese, and molybdenum—

have geologic sources, and others—nitrate, pesticides, soil fumigants, 

solvents—have sources associated with human activities. Contaminants from 

geologic sources were more commonly detected in drinking water sampled 

in the Columbia Plateau and Snake River Plain than on Oahu, whereas 

contaminants related to human activities were more commonly detected in 

drinking water on Oahu.
Contaminants related to human activities

Columbia Plateau Snake River Plain Oahu

EXPLANATION

Radon (G)

Arsenic (G)

Manganese (G)

Molybdenum (G)

(G) Contaminant from geologic sources

 Contaminant related to human activities

(N) Nutrient

(P) Pesticide

(F) Fumigant

(S) Solvent

Percentage of wells in which at least one contaminant was detected above a human-health benchmark

0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100

Nitrate (N)

Dieldrin (P)

1,2-dibromoethane (ethylene

dibromide, EDB) (F)

1,2-dichloropropane (DCP) (F)

Trichloroethylene (TCE) (S)

Contaminants from geologic sources

Radon greater than 

300 picocuries per liter

Radon greater than 

4,000 picocuries per liter

Greater than a human-health 

benchmark for drinking water

Greater than 1/10th of a human-health

benchmark for drinking water

Figure 5-1.

Radon, arsenic, manganese, and molybdenum 

from geologic sources are present at levels of 

concern for human health in some groundwater 

used for drinking 

Concentrations of contaminants from geologic sources—

 radon, arsenic, manganese, and molybdenum —were assessed 

by comparing them to a concentration equivalent to one-tenth 

of the human-health benchmark for that specific contaminant 

(see sidebar, Human-health benchmarks and other guidelines 

used in this assessment, p. 36). A higher percentage of samples 

from the Columbia Plateau and the Snake River plain contained 

at least one constituent at a concentration exceeding one-tenth 

of the human-health benchmark than from Oahu (fig. 5 –1). 

Radon exceeded its proposed U.S. Environmental Protection 

Agency (USEPA) Maximum Contaminant Level (MCL) of 

300 picocuries per liter (pCi/L) in more than 70 percent of 

samples from the Columbia Plateau and Snake River Plain, but 

exceeded its alternate proposed USEPA MCL of 4,000 pCi/L in 

Figure 5–1. Water from some drinking-water wells sampled in the Columbia Plateau, Snake 

River Plain, and Oahu aquifers contained at least one of these contaminants above its humanhealth benchmark. Contaminants from geologic sources were more frequently detected in 

drinking-water samples from the Columbia Plateau and Snake River Plain aquifers than in 

samples from Oahu. Conversely, several contaminants related to human activities were more 

frequently detected in drinking-water samples from Oahu than from the Columbia Plateau or 

Snake River Plain.

less than 2 percent of samples from these aquifers (table 5 –1). 

The two MCLs for radon were proposed by USEPA in 

1999—the lower MCL is for public water systems in States 

without programs to mitigate risk from other sources of radon, 

and the higher level is for water systems in areas that do have 

multimedia mitigation programs. In samples from Oahu, radon 

rarely was measured at a concentration that exceeded even 

the lower proposed MCL. Arsenic and manganese exceeded 

a human-health benchmark in less than 7 percent of samples 

from either the Columbia Plateau or the Snake River Plain, 

and none exceeded the benchmark on Oahu. Arsenic, however, 

was detected at a concentration greater than 1 microgram per 

liter (μg/L), one-tenth of the USEPA MCL, in about three 

of every four wells sampled in the Columbia Plateau and 

Snake River Plain. Molybdenum exceeded its Health-Based 

Screening Level (HBSL) in 7 percent of samples from the 

Snake River Plain, but in no samples from either the Columbia 

Plateau or Oahu—in fact, exceedance of even one-tenth of the 

HBSL was rare in samples from these two aquifers.

34 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 5

Table 5–1. Some samples of drinking water from domestic and public-supply wells located in the Columbia Plateau and Snake River 

Plain and on Oahu contained at least one of these nine contaminants at concentrations greater than human-health benchmarks. 

Contaminants from geologic sources and nitrate were more commonly detected above human-health benchmarks in drinking water 

from the Columbia Plateau and Snake River Plain, but were rarely detected above human-health benchmarks on Oahu. 

[pCi/L, picocuries per liter; MCL, Maximum Contaminant Level established by the U.S. Environmental Protection Agency (USEPA); μg/L, micrograms 

per liter; HBSL, Health-Based Screening Level developed by the U.S. Geological Survey (USGS) for constituents that have no MCL; Nitrate, nitrite plus 

nitrate as nitrogen; mg/L, milligrams per liter; shaded cells indicate water from at least one percent of wells exceeded the human-health benchmark]

Type of

drinkingwater

contaminant

Constituent

Human-health

benchmark*

(HHB)

Columbia Plateau Snake River Plain Oahu

Number of 

drinkingwater wells 

sampled†

Percent of

drinkingwater well 

samples 

exceeding 

HHB†

Number of

drinkingwater wells 

sampled†

Percent of

drinkingwater well 

samples 

exceeding 

HHB†

Number of

drinkingwater wells 

sampled†

Percent of

drinkingwater well 

samples

exceeding 

HHB†

Geologic

source

Radon

300 pCi/L Proposed 

MCL‡ 89 81 92 72 30 3

4,000 pCi/L

Proposed

alternative 

MCL‡

89 1 92 1 30 0

Arsenic 10 μg/L MCL 29 7 117 3 30 0

Manganese 300 μg/L HBSL 93 2 178 4 30 0

Molybdenum 40 μg/L HBSL 29 7 59 0 30 0

Human

source

Nitrate 10 mg/L MCL 93 17 179 3 30 0

Dieldrin 0.002 μg/L HBSL 

low§ 186 1 176 0 29 10

1,2-dibromoethane 

(ethylene dibromide, EDB)

0.05 μg/L MCL 64 3 61 0 30 0

1,2-dichloropropane 

(DCP) 5 μg/L MCL 93 0 65 2 30 0

Trichloroethylene 

(TCE) 5 μg/L MCL 93 0 65 0 30 3

*Human-health benchmarks are concentrations of constituents in drinking water that may be of potential concern for human health, if exceeded. The 

USEPA has established MCLs for some constituents.(95) The USGS has established HBSLs for additional constituents that do not have MCLs established.(96)

†Domestic and public-supply drinking-water wells were sampled in the Columbia Plateau and Snake River Plain (and used in this table), but only 

public-supply wells were sampled on Oahu. 

‡There is currently no federally enforced drinking-water standard for radon. The USEPA has proposed to require community water suppliers to 

provide water with radon levels no higher than 4,000 pCi/L, which contributes about 0.4 pCi/L of radon to the air in your home from showering and other 

household uses. Under the proposed regulation, States that choose not to develop enhanced indoor air programs will be required to reduce radon levels 

in drinking water to 300 pCi/L. This amount of radon in water contributes about 0.03 pCi/L of radon to the air in your home. (See http://water.epa.gov/

lawsregs/rulesregs/sdwa/radon/basicinformation.cfm and http://water.epa.gov/lawsregs/rulesregs/sdwa/radon/regulations.cfm.)

§The HBSL for dieldrin is a range of values. The low value (0.002 μg/L) corresponds to a one in one million cancer risk; the high value (0.2 μg/L) 

corresponds to a one in ten thousand cancer risk. 

Chapter 5: Quality of the Groundwater Resource Used for Drinking 35
Human-health benchmarks and other guidelines used in this assessment 

Concentrations of constituents measured for this assessment were compared to human-health benchmarks to place study 

findings in the context of human health. The benchmarks are threshold concentrations in water above which the concentration of a 

contaminant in drinking water could adversely affect human health. Human-health benchmarks were available for about two-thirds 

of the 200 constituents and properties measured for this assessment (appendix 2). Two types of human-health benchmarks were 

used: U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) (http://water.epa.gov/drink/contaminants/

index.cfm) and U.S. Geological Survey (USGS) Health-Based Screening Levels (HBSLs). MCLs are legally enforceable drinkingwater standards that specify the maximum permissible level of a constituent in water that is delivered to any user of a public water 

system(26) (values used in this report were current as of February 2012). Some States, such as Hawaii, have set more stringent MCLs 

than the USEPA for some constituents. Although MCLs are used to regulate the quality of drinking water only from public-supply 

sources, they also are useful for evaluating the quality of water from domestic and monitoring wells. An MCL was available for 

42 of the constituents measured. For some constituents for which an MCL has not been established, the USGS, in collaboration 

with the USEPA and others, developed non-enforceable HBSLs by using standard USEPA methods for establishing drinking-water 

guidelines and current toxicity information(94, 96, 98) (values used in this report were current as of February 2012; see http://water.usgs.

gov/nawqa/HBSL). An HBSL was available for 94 constituents measured. Radon has neither an MCL nor an HBSL, but two MCLs have 

been proposed. Copper and lead have USEPA action levels rather than an MCL.

In addition to human-health benchmarks, non-health-based guidelines—Secondary Maximum Contaminant Levels (SMCLs)—

were available for some of the constituents measured in this assessment. The SMCLs are non-enforceable guidelines for concentrations of “nuisance” constituents in drinking water that can cause unwanted cosmetic effects, such as skin or tooth discoloration; 

aesthetic effects, such as unpleasant taste, odor, or color; or technical effects, such as corrosion or sedimentation of plumbing or 

reduced effectiveness of water treatment.(99)

Concentrations greater than one-tenth of a human-health benchmark were used in this assessment to indicate which contaminants 

occurred, either individually or as mixtures, at concentrations that approach those of potential concern for human health, and to identify 

contaminants that might warrant additional monitoring and study. The criterion of one-tenth of a benchmark is consistent with various 

State and Federal practices for reporting contaminant occurrence in groundwater and for identifying contaminants of potential humanhealth concern (for example, see U.S. Environmental Protection Agency(92); New Jersey Department of Environmental Protection(100)).

Screening-level assessments, such as this one, provide perspective on the potential relevance of detected contaminants to 

human health and can help in planning future studies.(94) They are not designed to evaluate specific effects of contaminants on human 

health and are not a substitute for comprehensive risk assessments. It is important to note that occurrence of a contaminant at a 

concentration greater than its benchmark does not mean that adverse effects are certain to occur, because the benchmarks are 

conservative (protective) and source-water samples were collected prior to any treatment or blending that could alter contaminant 

concentrations in finished drinking water. There are water-treatment options, such as charcoal filtration, that can be used to lower 

the concentration of the contaminant to below the benchmark before the water is consumed.

Photographs by Michael G. Rupert, USGS

This makeshift chemigation system supplies fertilizers and (or) pesticides 

to irrigation water pumped by this large-capacity irrigation well. The water 

is then applied to the fields. Groundwater contamination can occur if a 

backflow preventer is not properly installed in the chemigation system or 

if the chemicals are applied at rates in excess of what the plants can use.

Dairies (foreground) and agricultural fields (background) can be 

major sources of nitrate to groundwater.

36 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
What is a contaminant?

Contaminants have a wide range of sources, both manmade and geologic. Most organic chemicals in groundwater that 

are of concern for human health are manmade. In contrast, most inorganic constituents in groundwater have geologic or 

other natural sources, although their concentrations in groundwater may be altered by human activities, such as irrigation and 

groundwater pumping. Some contaminants have both manmade and natural sources. For example, nitrate in groundwater has 

many natural sources, but nitrate concentrations in groundwater underlying agricultural and urban areas commonly are higher 

than in other areas because of contributions from sources associated with human activities.

But what exactly is a contaminant? The word means different things to different people. For example, a contaminant is 

defined by the Safe Drinking Water Act (SDWA) as “any physical, chemical, biological, or radiological substance or matter in 

water” (see http://www.epw.senate.gov/sdwa.pdf). This broad definition of contaminant includes every substance that may be 

found dissolved or suspended in water—everything but the water molecule itself. This is not a very practical definition because 

this would imply that all water is “contaminated.” Pure water that has nothing dissolved in it does not occur naturally—not even 

rainfall is pure water, because it contains, at a minimum, some dissolved gases.

The U.S. Environmental Protection Agency defines a contaminant as “Any physical, chemical, biological, or radiological 

substance or matter that has an adverse effect on air, water, or soil” (see http://epa.gov/region04/superfund/qfinder/glossary.html).

This definition is more practical and allows both manmade constituents and those with geologic sources in water to be defined 

as contaminants. However, it does not define what “adverse” means, and what may be adverse in one way might be beneficial 

in another. In this circular, a contaminant is defined as any physical, chemical, biological, or radiological substance or matter in 

groundwater that is manmade or that impairs the use of water for its intended purpose. Impairment is determined by comparing 

a measured concentration to benchmarks or guidelines. By this definition, all manmade compounds, such as pesticides and 

volatile organic compounds, are contaminants because they do not occur naturally in groundwater. If a constituent with 

a geologic source, such as arsenic, occurs in drinking water at a concentration above its human-health benchmark, it also 

is considered a contaminant.

Chapter 5

Nitrate exceeded human-health benchmarks 

more frequently in groundwater from drinkingwater supply wells in the Columbia Plateau 

than in groundwater in the Snake River Plain 

or on Oahu, whereas pesticides exceeded 

human-health benchmarks more frequently in 

groundwater from drinking-water supply wells 

on Oahu than in groundwater in the Columbia 

Plateau and Snake River Plain 

Nitrate, a contaminant largely related to human activities, 

was frequently measured at a concentration exceeding 

one-tenth of the USEPA MCL in some drinking-water samples 

from all three aquifers. Nitrate exceeded the USEPA MCL 

of 10 mg/L as nitrogen most frequently in samples from the 

Columbia Plateau —in 17 percent of wells — but in only 

3 percent of samples from the Snake River Plain and no 

samples from Oahu. In contrast, pesticides, also associated 

with human activities, exceeded one-tenth of a human-health 

benchmark more often in some samples from Oahu than in 

samples from either the Columbia Plateau or the Snake River 

Plain. Concentrations of the pesticide dieldrin, the use of 

which was banned in 1987, exceeded the lower human-health 

benchmark value of 0.002 μg/L in 10 percent of drinking-water 

samples from Oahu, but in only 1 percent of drinking-water 

samples from the Columbia Plateau and in no samples from 

the Snake River Plain. The human-health benchmark for 

dieldrin is a range of values; the low value (0.002 μg/L) corresponds to a one in one million cancer risk, and the high value 

(0.2 μg/L) corresponds to a one in ten thousand cancer risk.

Concentrations of several constituents in water 

from drinking-water supply wells exceeded 

USEPA guidelines for taste, odor, and corrosivity

Water from some drinking-water wells in all three study 

areas contained concentrations of dissolved solids, sulfate, 

chloride, manganese, and iron above Secondary Maximum 

Contaminant Levels (SMCLs) established by the USEPA to 

avoid problems, such as unpleasant taste and odor, staining, 

and corrosion, which make drinking water undesirable 

(fig. 5–2; table 5 –2).(99) Dissolved solids (also called total 

dissolved solids, or TDS) was the constituent that most 

commonly exceeded its SMCL (500 mg/L) in drinking water 

in all three study areas, and the percentage of exceedances was 

similar to that in the Nation as a whole.(97) Dissolved solids is 

a measure of all inorganic and organic constituents dissolved 

in a water sample, and is an overall indication of the quality 

of drinking water. 

Chapter 5: Quality of the Groundwater Resource Used for Drinking 37
Table 5 –2. SMCLs were exceeded for dissolved solids, sulfate, chloride, manganese, and iron in water from domestic and publicsupply wells sampled in the Columbia Plateau and Snake River Plain and on Oahu. SMCLs are non-enforceable guidelines for constituents

that may cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as an unpleasant taste, odor, or color) 

in drinking water. Water sampled from domestic and public-supply wells on Oahu generally had fewer exceedances of SMCLs than 

water sampled in the Columbia Plateau or Snake River Plain. 

[SMCL, Secondary Maximum Contaminant Level established by the U.S. Environmental Protection Agency; mg/L, milligrams per liter; μg/L micrograms per liter]

Constituent SMCL 

value

The Columbia Plateau, 

Snake River Plain,

and Oahu combined

Columbia Plateau Snake River Plain Oahu

Number of 

drinkingwater wells 

sampled*

Percent of 

drinkingwater well 

samples 

exceeding 

SMCL

Number of 

drinkingwater wells 

sampled*

Percent of 

drinkingwater well 

samples 

exceeding 

SMCL

Number of 

drinkingwater wells 

sampled*

Percent of 

drinkingwater well 

samples 

exceeding 

SMCL

Number of 

drinkingwater wells 

sampled*

Percent of 

drinkingwater well 

samples 

exceeding 

SMCL

Dissolved 

solids

500 mg/L 241 18 92 16 119 22 30 10

Sulfate 250 mg/L 300 2 92 4 178 0 30 0

Chloride 250 mg/L 300 1 92 1 178 0 30 3

Manganese 50 μg/L 301 7 93 10 178 6 30 0

Iron 300 μg/L 301 3 93 6 178 1 30 0

*Domestic and public-supply drinking-water wells were sampled in the Columbia Plateau and Snake River Plain (and used in this table), but only publicsupply wells were sampled on Oahu. 

Columbia Plateau Snake River Plain Oahu

Percentage of wells in which constituent was detected above a Secondary Maximum Contaminant Level (SMCL) 

0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25

Percentage of wells with concentrations greater than the SMCL 

Dissolved solids

Sulfate

Chloride

Manganese

Iron

Figure 5-2.

Figure 5–2. Concentrations of dissolved solids, sulfate, chloride, manganese, and iron 

exceeded the USEPA SMCLs in samples from drinking-water (domestic and public supply) 

wells in the Columbia Plateau and Snake River Plain and on Oahu. SMCLs are non-enforceable 

guidelines for constituents that can cause drinking water to have an unpleasant taste or smell, 

stain plumbing fixtures and laundry, or corrode pipes.

Elevated concentrations of dissolved solids are not a health hazard, but can indicate the potential 

presence of other constituents that, at elevated concentrations, are a concern for human health. 

38 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 6: Understanding Where and Why 

Key Contaminants Occur in Groundwater

Chapter 6

Photograph copyright Kestrel Aerial 

Services, Inc., ©www.kestrelaerial.

com, used with permission

Constituents from geologic sources and contaminants from human 

activities were commonly detected in groundwater in many areas 

of the Columbia Plateau and Snake River Plain and on Oahu. The 

occurrence of constituents from geologic sources is related to the basin-fill 

sediments and basaltic rocks that make up the aquifers, and the geochemical 

conditions within those aquifers. The occurrence of nitrate, pesticides, and 

volatile organic compounds (VOCs) is related to human activities, primarily 

agricultural, but also urban and military.

There were five times as many dairies 

in the Snake River Plain in 2007 as there were 

in 1987. Many of these dairies house several 

thousand cattle, and resemble industrial facilities 

in their planning and design. Three lagoons, which 

store liquid wastes, are shown here—two in the front 

of the facility and one in the back. The solid and liquid 

wastes can be applied as fertilizer to the surrounding 

agricultural lands, but groundwater contamination 

can result if those waste materials are applied 

in excess of what can be taken up by plants 

(agronomic rate). Groundwater contamination 

also can result from infiltration of nitrateladen leachate from the facility itself. 

This chapter describes 

the sources of and factors

that affect constituents 

from geologic sources 

and contaminants from 

human activities in 

the Columbia Plateau, 

Snake River Plain, and 

Oahu aquifers. 
See sidebar, Boxplots, p. 41

USEPA

proposed MCL

(300 pCi/L)

Outlier at

7,200 pCi/L

not shown

Radon activity, in picocuries per liter

3,000

2,500

2,000

1,500

1,000

500

0

Columbia 

Plateau

Snake River 

Plain

Oahu

Outlier at

4,100 pCi/L

not shown

Radon activity

Figure 6-1.

Rn

86

222

Concentrations of Constituents from Geologic Sources—Radon, Arsenic, 

Molybdenum, and Manganese—Exceeded Human-Health Benchmarks in 

Water From Some Wells in the Columbia Plateau and Snake River Plain, 

but Not in Water From Wells on Oahu 

Concentrations of constituents from geologic sources in groundwater are related 

to the materials that make up the aquifer and to the geochemical conditions in the 

aquifer. For example, in the Snake River Plain, elevated radon activities in groundwater 

are associated with uranium-rich aquifer minerals contained in basin-fill sediments. 

Elevated concentrations of radon, arsenic, molybdenum, and manganese were detected 

in water from some wells in the Columbia Plateau and Snake River Plain, but not in 

water from wells on Oahu, because the aquifer materials and geochemical conditions 

on Oahu do not promote mobilization of these constituents in groundwater. 

Radon activities were much higher in groundwater from the Columbia Plateau 

and Snake River Plain aquifers than in groundwater from the Oahu aquifer

Radon is a naturally occurring, radioactive, colorless, odorless, tasteless gas that is formed 

as part of the normal radioactive decay of uranium, a radioactive element. Most rocks contain 

some uranium, but some types of rocks—such as dark shales and granites—have higher-thanaverage uranium contents. Radon exposure is the second most frequent cause of lung cancer, 

after cigarette smoking, causing between 15,000 and 22,000 lung cancer deaths each year in the 

United States.(27, 28) One of the greatest risks for radon exposure from groundwater occurs when 

radon in water is released into the air during showering, which poses a much greater risk than 

ingesting the same water.

The USEPA has proposed both an MCL of 300 pCi/L and an alternative MCL of 

4,000 pCi/L for radon in public water systems.(29) The lower proposed MCL for radon applies to 

States and public water systems that have not established programs to address health risks from 

radon in indoor air; the higher proposed alternative MCL applies to States and public water 

systems that have established such programs.(29, 30)

Radon was detected in water from more than 99 percent of the wells sampled in the 

Columbia Plateau and Snake River Plain and on Oahu. Radon activities exceeded the lower 

proposed USEPA MCL of 300 pCi/L in about 75 percent of samples from the Columbia Plateau 

and about 50 percent of samples from the Snake River Plain aquifers (fig. 6–1), although 

the proposed alternative USEPA MCL of 4,000 pCi/L was exceeded in water from only 

two wells—one in the Columbia Plateau and one in the Snake River Plain. On Oahu, radon 

activities in water from only one well exceeded the lower proposed USEPA MCL of 300 pCi/L, 

and none exceeded the higher proposed alternative USEPA MCL of 4,000 pCi/L. 

Figure 6–1. Radon activities 

in water were greater than 

the proposed USEPA MCL of 

300 pCi/L from most of the wells 

sampled in the Columbia Plateau 

and Snake River Plain, but only 

one well sampled on Oahu. 

Radon activities in water were 

above the proposed alternative 

USEPA MCL of 4,000 pCi/L from 

only one well sampled in the 

Columbia Plateau and one well 

in the Snake River Plain.

40 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005

Radon is an element that 

results from the radioactive 

decay of uranium in aquifer 

materials. Radon in drinking 

water is associated with lung 

cancer and stomach cancer.
Boxplots 

Boxplots are used to illustrate how results are distributed within a group. The “box” ranges from the 25th to the 75th percentile 

and represents 50 percent of the data. The horizontal line in the middle of the box is the median value— one-half of the values in the 

group are greater than the median and one-half are less. 

Percentiles describe the percentage of values in a group that are 

less than the given value: 25 percent of the values in a group are less than 

the 25th percentile; 75 percent of the values in a group are less than the 

75th percentile. The median is also the 50th percentile.

If, for example, the 75th percentile for the measured concentration 

of a contaminant in a group of wells is equal to the human-health 

benchmark for that contaminant, then 75 percent, or three-fourths, of the 

wells have a concentration of that contaminant less than the benchmark, 

and 25 percent, or one-fourth, have a concentration greater than the 

benchmark.

The “whiskers” (vertical lines) in these figures extend to the 10th and 

90th percentiles; box and whiskers together represent 80 percent of the 

data. Values greater than the 90th or less than the 10th percentiles are 

shown as individual points (outliers).

EXPLANATION

25th percentile

10th percentile

75th percentile

50th percentile

 (median)

90th percentile

Outlier

Outlier

50 percent of data

fall within the box 

80 percent of data fall 

 between endpoints of whiskers 

Otton and others (89)

Chapter 6

Radon gas (represented by red dots) dissolved in groundwater 

can move with water from a domestic well into household plumbing. 

When the water is aerated at faucets, radon can be released into 

the air and inhaled while people shower or run faucets.

The dissolution and 

weathering of rocks 

and minerals that 

make up aquifers 

and soils contribute 

constituents to 

groundwater. Sources 

of constituents that, at 

high concentrations, 

are of concern for 

human health include 

pyrolusite (top), a 

manganese oxide 

mineral, and granite 

(bottom), an igneous 

rock that can be 

enriched in uranium.

Photographs by Andrew Silver, USGS

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 41

One of the greatest risks for radon exposure from 

groundwater with high radon concentrations 

occurs when radon in water is released into 

the air during showering, which poses a much 

greater risk than ingesting the same water.
Lithology within well 

network (graph)

Snake River Plain 

study areas (map)

Basin-fill sediments

Minidoka area

A&B area

Eden area

Jerome/Gooding area

Snake River Plain

Tributary valleys

Jackson valley

Basaltic rock

EXPLANATION

See sidebar, Boxplots, p. 41

Agricultural land use Mixed land use

Outlier at

7,200 pCi/L

not shown

Radon activity, in picocuries per liter

3,000

2,500

2,000

1,500

1,000

500

0

USEPA 

proposed

MCL

(300 pCi/L)

Radon activity

Snake River Plain, domestic wells

Tributary valleys, domestic wells

Jackson valley, domestic 

and public-supply wells

Minidoka area, domestic wells

Eden area, domestic wells

A&B area, domestic wells

Jerome/Gooding area, 

domestic wells

Figure 6-2.

Snake River Plain

N

Elevated radon activities in groundwater in the Snake River Plain were associated with 

uranium-rich aquifer minerals contained in basin-fill sediments (fig. 6–2). Radon activities 

in groundwater sampled from the basalt aquifer in the A&B, Eden, and Jerome/Gooding 

areas of the Snake River Plain were low because the basaltic rocks contain relatively low 

concentrations of uranium. Radon activities in groundwater sampled in the tributary valleys 

and the Jackson valley were higher because the sedimentary aquifers contain sediments eroded 

from the surrounding mountains, which are partly composed of granitic and metamorphic rocks 

that contain higher concentrations of uranium than does basalt. Groundwater sampled from the 

sedimentary aquifer in the Minidoka area has higher radon activities than groundwater sampled 

from the underlying basalt aquifer because the sediments were deposited by the Snake River, 

which transported uranium-bearing sediments from distant places. Groundwater sampled on 

Oahu had much lower radon activities than did groundwater sampled in the Columbia Plateau 

or the Snake River Plain because all the groundwater sampled on Oahu was withdrawn from 

the basalt aquifer.

Figure 6–2. Radon 

activities are relatively low 

in groundwater samples 

collected from the basalt 

aquifer in the Snake River 

Plain because the basaltic 

rocks have relatively low 

concentrations of uranium. 

Radon activities were 

higher in groundwater 

sampled from the basinfill aquifers because they 

contain uranium-rich 

granitic and metamorphic 

sediments that eroded from 

the surrounding mountains.

Constituents from geologic 

sources, such as radon, can 

occur in groundwater at 

concentrations of potential 

human-health concern 

even in undeveloped areas 

where contamination 

typically is not expected, 

such as near Grand Teton, 

Wyoming, where this well is 

located. Traditional wellhead 

protection approaches 

designed to reduce sources 

of manmade contaminants 

to groundwater generally 

are not designed to prevent 

groundwater contamination 

from geologic sources.

Photograph by Michael G. Rupert, USGS

42 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 6

Concentration, in micrograms per liter

10

8

6

4

2

0

Alkalinity, as calcium carbonate, 

in milligrams per liter

100 200 300 400

LOWESS (locally weighted 

scatterplot smoothing) line

Arsenic concentration

Figure 6-3.

As

33

75

Arsenic concentrations exceeded the USEPA MCL more commonly in 

groundwater from the Columbia Plateau and Snake River Plain than in 

groundwater from Oahu

Arsenic is a nonmetallic trace element in rocks and soils that has many geologic sources, 

including sulfide minerals and geothermal deposits; it also has been used in some pesticides. 

Arsenic is toxic to humans and, in drinking water, can contribute to skin, bladder, and lung 

cancers.(31) Arsenic in drinking water is one of the leading environmental causes of cancer 

mortality in the world.(32)

Arsenic concentrations exceeded the USEPA MCL of 10 μg/L more commonly in groundwater from the Columbia Plateau (24 percent of wells) than in groundwater from the Snake 

River Plain (5 percent of wells) or Oahu (no wells). In fact, arsenic concentrations did not 

exceed one-tenth of the USEPA MCL in water from any wells sampled on Oahu, indicating 

that arsenic in groundwater on the island is not a human-health concern. McMahon and 

Chapelle(33) reported that the Columbia Plateau and Snake River Plain had some of the highest 

arsenic concentrations of the 15 Principal Aquifers studied by NAWQA, but few investigations 

have been conducted into the cause of those elevated arsenic concentrations.

The factors related to elevated arsenic concentrations in groundwater are complex and 

are related to geochemical conditions in the aquifer. The most common cause of elevated 

arsenic concentrations in groundwater is the release of arsenic from iron oxides in the aquifer 

materials.(34) Elevated arsenic concentrations can occur if the groundwater has low dissolved 

oxygen (anoxic conditions) or high pH (alkaline conditions); elevated arsenic concentrations 

also can occur if the arsenic has been concentrated by evaporation.(35) In the Columbia Plateau 

and Snake River Plain, all pH levels were above 7 and the groundwater was oxic. Under these 

conditions, arsenic can be released from aquifer materials by pH-driven desorption.(35) Elevated 

arsenic concentrations in groundwater from the Snake River Plain also were associated with 

elevated concentrations of dissolved solids, silica, and alkalinity (fig. 6–3), indicating that 

arsenic occurrence also could be related to geochemical changes resulting from evaporative 

concentration.(34) Another possibility is that that elevated alkalinity in irrigation recharge may 

be desorbing arsenic.(36) The low arsenic concentrations in groundwater from Oahu probably 

are related to the humid climate, iron-rich soils, and oxic condition of the groundwater. The 

humid climate of Oahu would preclude evaporative concentration, and the iron-rich soils and 

oxic conditions would tend to bind arsenic in the soil and aquifer materials. Although sodium 

arsenate was widely used as a pesticide on sugarcane on the Hawaiian Islands from about 

1913 to 1945, the arsenic is tightly bound in the iron oxides in the soil and, therefore, unlikely 

to be released to groundwater.(37)

Figure 6–3. In the Snake River 

Plain, arsenic concentrations 

tend to increase when 

alkalinity increases, indicating 

that elevated arsenic 

concentrations may result 

from concentration of arsenic 

(and other constituents, such 

as dissolved solids and silica) 

during evaporation.

Arsenic is a trace 

element found in aquifer 

materials and can also 

come from pesticide 

application or industrial 

waste. Long-term 

exposure to arsenic 

in drinking water is 

related to elevated 

risks of cancer and 

skin damage.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 43
LOWESS (locally weighted 

scatterplot smoothing) line

5

4

3

2

1

0

Concentration, in micrograms per liter

pH, in standard units

7.0 7.5 8.0 8.5

Molybdenum concentration

Figure 6-4.

Mo

42

96

Mn

25

55

Concentrations of molybdenum were much higher in groundwater in the 

Columbia Plateau than groundwater in the Snake River Plain or on Oahu 

Molybdenum is an essential trace element for animals and humans,(38) but long-term 

consumption of drinking water with high concentrations of molybdenum can affect human 

health. The adverse effects include enlarged liver, gastrointestinal and kidney disorders, and a 

gout-like disease that causes joint pain in the 

hands and feet.(39)

Concentrations of molybdenum in water 

from 4 of 76 wells (5 percent) sampled in the 

Columbia Plateau were above the humanhealth benchmark of 40 μg/L; molybdenum 

concentrations were much lower in 

groundwater in the Snake River Plain and 

Oahu, where there were no exceedances of 

the benchmark. Similar to arsenic, the factors 

related to molybdenum concentrations in 

groundwater are complex and are related to 

geochemical conditions in the aquifer. The 

cause of the elevated molybdenum concentrations in groundwater in the Columbia 

Plateau is unknown. In the Snake River 

Plain, where molybdenum concentrations 

are much lower, concentrations increase with 

increasing pH levels (fig. 6–4), suggesting 

release of molybdenum from aquifer materials 

by pH-driven desorption.(35)

Manganese in groundwater rarely occurred at elevated concentrations 

because groundwater in all three study areas mostly is oxic

Manganese, which is found in many types of rocks and soils, is an essential trace element 

for human health. Studies indicate that exposure to extremely high levels of manganese could 

produce undesirable effects on brain development in children, including changes in behavior 

and impairing the ability to learn and retain information.(40) High manganese concentrations in 

drinking water also can harm the livers of adults and children.(41)

Manganese concentrations exceeded the human-health benchmark of 300 μg/L in water 

from only two wells (1 percent) sampled in the Columbia Plateau and from only seven wells 

(3 percent) sampled in the Snake River Plain; manganese did not exceed the benchmark in 

water from any of the wells sampled on Oahu. In addition to having some human-health 

concerns, manganese can cause black staining on plumbing fixtures and can give drinking 

water an unpleasant taste, so it has an SMCL of 50 μg/L. Concentrations of manganese in water 

from 7 percent of the wells sampled in the Columbia Plateau and 6 percent of the wells sampled 

in the Snake River Plain exceeded the SMCL. Water from none of the wells sampled on Oahu 

exceeded the SMCL.

Manganese concentrations in groundwater rarely exceeded the human-health benchmark 

or the SMCL because most groundwater in the three study areas is oxic (fig. 4–3). Elevated 

manganese concentrations typically occur under anoxic conditions (low dissolved-oxygen 

concentrations) because manganese-containing minerals dissolve more readily in anoxic water. 

All of the samples with manganese concentrations that exceeded the human-health benchmark 

or the SMCL were collected in groundwater under mixed oxic/anoxic or anoxic conditions. 

Ayotte and others(35) also noted the association between elevated manganese concentrations and 

anoxic conditions in groundwater from the Columbia Plateau and Snake River Plain. 

Molybdenum is a trace 

element that is naturally 

present, usually at low levels, 

in aquifer materials. Trace 

amounts of molybdenum 

are necessary for human 

health, but consumption 

of molybdenum above the 

human-health benchmark of 

40 μg/L can lead to enlarged 

liver, gastrointestinal and 

kidney disorders, and a goutlike disease that causes joint 

pain in the hands and feet.

Manganese is a trace 

element found in aquifer 

materials and some industrial 

wastes. Manganese has a 

human-health benchmark 

of 300 μg/L because of 

neurological effects.

Figure 6–4. Molybdenum concentrations 

in groundwater in the Snake River Plain are 

associated with elevated pH levels, indicating 

that molybdenum could be released from the 

aquifer materials by pH-driven desorption.

44 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
What is nitrate? 

Nitrogen, which is essential for plant growth, occurs in several forms, including nitrate, ammonia, and 

organic nitrogen, but most of the nitrogen in groundwater is in the form of nitrate. Efforts to increase food 

production to meet the demands of a growing global population have greatly increased the use of nitrogen 

fertilizers, which has, in turn, increased the occurrence and concentrations of nitrate in groundwater.

Nitrate has some natural sources, but most of the nitrate in the environment has been contributed 

by human activities, including production and application of nitrogen fertilizers, cultivation of nitrogenfixing crops, animal waste disposal, wastewater discharge, industrial discharges, and combustion of 

fossil fuels.(42, 43) Human activities have nearly doubled the rate of nitrogen inputs into the terrestrial nitrogen 

cycle, which is reflected in higher nitrate concentrations in groundwater.

What would nitrate concentrations be in groundwater if there were no 

effects of human development? 

Nitrate occurs naturally, but the largest sources of nitrate are from human activities. Studies in the 

Snake River Plain(14, 44) and on Oahu(16, 45, 46) indicate that natural “background” nitrate concentrations in 

the study areas are less than 1 mg/L as nitrogen. This finding is consistent with information gathered by 

NAWQA in many areas of the Nation that have little or no development: background concentrations of 

nitrate in groundwater in many areas across the United States are less than 1 mg/L.(25, 93)

Chapter 6

Nitrate in the Columbia Plateau, Snake River Plain, and Oahu aquifers

Nitrate concentrations in groundwater exceeded the USEPA MCL of 10 mg/L as nitrogen 

in 20 percent of the wells sampled in the Columbia Plateau and in 4 percent of the wells 

sampled in the Snake River Plain, but did not exceed the USEPA MCL in water from any of 

the wells sampled on Oahu. The highest concentrations of nitrate are associated with shallow 

groundwater in basin-fill sediments beneath agricultural land, such as in the Quincy-Pasco, 

Washington, area of the Columbia Plateau, where concentrations of nitrate in almost 

40 percent of the wells exceeded the USEPA MCL. Nitrate concentrations in groundwater have 

increased in the Snake River Plain from 1993 to 2005, where nitrogen inputs from fertilizers and 

cattle manure have been increasing since the 1950s; however, nitrate concentrations did not 

change in the Columbia Plateau from 1993 to 2002, and there is insufficient information on 

Oahu to determine if nitrate concentrations have changed since 2000. 

Lush crop growth in the desert 

environment of the Snake 

River Plain requires application 

of fertilizers, pesticides, and 

irrigation water, supplied here by 

roller-wheel spray lines watering 

this field. Fertilizers and pesticides 

can be carried to the water table 

by excess irrigation water. A low 

shield volcano, evidence of the 

volcanic environment that affects 

groundwater occurrence and 

movement in the area, lies in the 

far distance.

Photograph by Charles D. Hunt, Jr., USGS

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 45
Columbia Plateau Snake River Plain Oahu

Figure 6-6.

0 40 80 MILES

0 40 80 KILOMETERS

0 4 8 MILES

0 4 8 KILOMETERS

0 20 40 MILES

0 20 40 KILOMETERS

EXPLANATION

Nitrate concentration in groundwater, 

in milligrams per liter as nitrogen

<1

1 to 10

High: 100 percent >10

Low: 0 percent

Percentage of agricultural land 

within a 500-meter radius

N

N

N

See sidebar, Boxplots, p. 41

Concentration, in milligrams

per liter as nitrogen

Background

(1 mg/L)

40

35

30

25

20

15

10

5

0

Outlier at

58 mg/L 

not shown

Outlier at

59 mg/L 

not shown

Nitrate concentration

USEPA MCL (10 mg/L)

Columbia 

Plateau

Snake River 

Plain

Oahu

Figure 6-5.

The primary sources of elevated nitrate in the Columbia Plateau and 

Snake River Plain and on Oahu are associated with agricultural activities

Nitrate concentrations exceeded the USEPA MCL of 10 mg/L as nitrogen in water from 

20 percent of the wells sampled in the Columbia Plateau and in 4 percent of the wells sampled 

in the Snake River Plain, but did not exceed the USEPA MCL in water from any of the wells 

sampled on Oahu (fig. 6–5). More than 65 percent of all groundwater samples collected in the 

three study areas combined had nitrate concentrations above the background concentration 

of 1 mg/L as nitrogen; in the Columbia Plateau, water from 80 percent of the wells sampled 

had nitrate concentrations above the background concentration. The primary sources of 

elevated nitrate concentrations in groundwater from the Columbia Plateau and Snake River 

Plain above background concentrations are nitrogen fertilizers and animal manure.(44, 47, 48)

Elevated nitrate concentrations in groundwater on Oahu are the result of decades of agricultural 

activities, particularly in sugarcane fields.(46) Because the primary sources of elevated nitrate 

are associated with agricultural activities, elevated nitrate concentrations were most common 

in groundwater underlying agricultural lands (fig. 6–6).

Elevated concentrations of nitrate in drinking water are associated with several humanhealth concerns. Nitrate at concentrations above the USEPA MCL of 10 mg/L as nitrogen 

can cause low oxygen levels in the blood of infants (a disorder called methemoglobinemia, 

or blue-baby syndrome). Long-term exposure to nitrate at concentrations of 2 to 4 mg/L in 

drinking water has possible links to bladder and ovarian cancer,(49) and to a type of cancer 

called non-Hodgkins lymphoma.(50)

Figure 6–6. Elevated nitrate concentrations 

in groundwater coincide with agricultural 

areas in the Columbia Plateau and Snake 

River Plain and on Oahu.

Figure 6–5. Groundwater in the 

Columbia Plateau had higher nitrate 

concentrations than groundwater 

in the Snake River Plain and 

Oahu aquifers; concentrations in 

20 percent of the wells sampled 

exceeded the USEPA MCL of 

10 mg/L as nitrogen. Groundwater 

on Oahu had the lowest nitrate 

concentrations, none of which 

exceeded the USEPA MCL. 

46 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 6

Lithology within well network (graph)

EXPLANATION

Columbia Plateau study areas and sampled wells (map)

Loess

Basin-fill sediments

Basin-fill sediments and basaltic rock 

Basaltic rock

Columbia public supply

Palouse area

Quincy-Pasco area, row crops and orchards

0 20 40 MILES

0 20 40 KILOMETERS

Columbia public supply

Palouse area, domestic wells

Palouse area, 

groundwater near water table

Quincy-Pasco row crops, 

domestic wells

Quincy-Pasco row crops, 

groundwater near water table

Quincy-Pasco orchards, 

domestic wells

Quincy-Pasco orchards, 

groundwater near water table

40

35

30

25

20

15

10

5

Concentration, in milligrams per liter as nitrogen

0

Mixed

land use

Agricultural land use

Outlier at

59 mg/L 

not shown

Nitrate concentration

USEPA MCL

(10 mg/L)

Columbia Plateau

Figure 6-7.

See sidebar, Boxplots, p. 41

Background

(1 mg/L)

N

Nitrate concentrations of groundwater in the 

Columbia Plateau are elevated 

The highest nitrate concentrations measured in groundwater from any localized area of the Columbia Plateau, 

Snake River Plain, or Oahu aquifers were from the QuincyPasco, Washington, area of the Columbia Plateau, where 

concentrations in almost one of every three wells sampled 

(30 percent) were above the USEPA MCL of 10 mg/L as 

nitrogen (fig. 6–7). Groundwater underlying two types of land 

use—irrigated row crops and orchards—was sampled in the 

Quincy-Pasco area. In addition, groundwater at two depths was 

sampled: shallow groundwater near the water table was sampled 

from monitoring wells, which withdraw groundwater from the 

basin-fill sediments, and deeper groundwater was sampled from 

domestic wells, which withdraw groundwater from a mixture 

of basin-fill sediments and basalt rock. Nitrate concentrations 

in almost 40 percent of the wells sampled near the water table 

underlying row crops in the Quincy-Pasco area exceeded the 

USEPA MCL of 10 mg/L as nitrogen. Nitrate concentrations 

in groundwater from the deeper domestic wells also underlying 

row crops were lower, but still, more than 30 percent of these 

samples contained nitrate at a concentration that exceeded the 

USEPA MCL. Nitrate concentrations also were elevated in 

groundwater underlying orchards in the Quincy-Pasco area, 

but were not as high as in groundwater underlying row crops.

Nitrate concentrations also were elevated in groundwater 

sampled in the Palouse, Washington, area of the Columbia 

Plateau. Two types of wells were sampled in the Palouse area: 

shallow monitoring wells that withdraw groundwater from 

near the water table in wind-blown silt (loess) sediments and 

deeper domestic wells that withdraw groundwater from the 

underlying basalt aquifer. Groundwater in shallow monitoring 

wells had higher nitrate concentrations than groundwater in 

the deeper domestic wells—nitrate concentrations of water 

from 20 percent of the wells sampled near the water table 

exceeded the USEPA MCL, whereas water from only one 

deeper domestic well exceeded the USEPA MCL. The crops 

grown in the Palouse area are dryland (non-irrigated) winter 

and spring wheat, barley, peas, and lentils. Although the crops 

are not irrigated, they are heavily fertilized, which has caused 

elevated nitrate concentrations.(47)

Groundwater sampled from parts of the Columbia Plateau 

with a mix of land uses had the lowest nitrate concentrations 

measured in the Columbia Plateau. This finding likely is the 

result of a smaller contribution of nitrate from agriculture, 

and because the wells are deeper than the wells sampled in 

agricultural land-use areas.

Figure 6 –7. Nitrate concentrations in groundwater 

sampled from basin-fill sediments underlying row 

crops in the Quincy-Pasco, Washington, area were the 

highest of those measured in the Columbia Plateau. 

Nitrate concentrations in almost 40 percent of the wells 

completed near the water table and in 33 percent of the 

deeper domestic wells sampled in the same area were 

above the USEPA MCL of 10 mg/L as nitrogen. 

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 47

Concentrations of nitrate in water from 

one-third of the wells sampled in the 

Quincy-Pasco, Washington, area exceeded 

the USEPA Maximum Contaminant Level.
Lithology within well 

network (graph)

Snake River Plain 

study areas (map)

Basin-fill sediments

Minidoka area

A&B area

Eden area

Jerome/Gooding area

Snake River Plain

Tributary valleys

Jackson valley

Basaltic rock

EXPLANATION

See sidebar, Boxplots, p. 41

Snake River Plain, domestic wells

Tributary valleys, domestic wells

Jackson valley, domestic 

and public-supply wells

Minidoka area, domestic wells

Eden area, domestic wells

A&B area, domestic wells

Jerome/Gooding area, 

domestic wells

Nitrate concentration, in

milligrams per liter as nitrogen

30

25

20

15

10

5

0

Agricultural land use Mixed land use

Outlier at

58 mg/L

not shown

Nitrate concentration

Figure 6-8.

USEPA MCL

(10 mg/L)

Background

(1 mg/L)

Snake River Plain

0 40 80 MILES

0 40 80 KILOMETERS

N

Concentrations of nitrate are high in groundwater in some areas of the

Snake River Plain, but in other areas are diluted by upwelling of deep, 

regional groundwater

The highest nitrate concentrations measured in groundwater in the Snake River Plain 

were in the Minidoka, Idaho, area, where the samples were collected from domestic wells that 

tap a shallow alluvial aquifer perched above the regional basalt aquifer (fig. 6–8). Water from 

more than one of every four wells sampled (over 25 percent) in the Minidoka area had nitrate 

concentrations above the USEPA MCL of 10 mg/L as nitrogen. Nitrate concentrations likely are 

elevated because of shallow depth to groundwater, permeable soils, and oxic groundwater conditions, which prevent the transformation of nitrate to harmless nitrogen gas (see chapter 4).(14)

In the deeper basalt aquifer underlying agricultural lands of the Snake River Plain, nitrate 

concentrations are elevated in the A&B area and decrease in the direction of groundwater 

flow (westward) to the Eden area and then the Jerome/Gooding area (fig. 6–8). The decrease 

in concentrations is caused by the geometry of the aquifer and the effect the geometry has on 

groundwater flow.(51)

Figure 6 –8. In the Snake River Plain, nitrate 

concentrations were highest in samples of 

groundwater from basin-fill sediments underlying 

agricultural lands in the southwestern part of the 

plain, where irrigation and cultivation of row crops, 

such as potatoes and sugar beets, are intensive. 

Nitrate concentrations decrease as groundwater 

flows from the A&B area, to the Eden area, and 

finally to the Jerome/Gooding area, as a result of 

the upwelling and mixing of nitrate-poor regional 

groundwater with nitrate-rich recharge from 

overlying agricultural lands. Nitrate concentrations 

in almost all wells sampled in the Jackson valley, 

in the eastern part of the study area, were below 

the background concentration of 1 mg/L as 

nitrogen because the source of the groundwater 

recharge mostly is runoff of snowmelt from 

an area with very little agriculture.

48 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 6

Regional groundwater flow

Modified from Garabedian(88)

7,000

6,000

5,000

4,000

3,000

2,000

1,000

Sea level

–1,000

Recharge through agricultural lands, largely from 

irrigation of fields and seepage losses from irrigation 

ditches and canals

Deep regional groundwater converges 

upward and mixes with contaminated irrigation 

recharge before discharging in the Thousand Springs

area along the Snake River

Recharge through rangeland 

and from stream and river 

infiltration to the plain 

farther east

A

Figure 6-9.

A’

Thousand 

Springs

Snake 

River

FEET

0 10 20 30 40 KILOMETERS

0 10 20 30 40 MILES

A’

A

EXPLANATION

Water with background nitrate concentrations

Water with elevated nitrate concentrations

Mixed water

Water table

Water flow direction

A A’ Line of section

Water table

Snake River Plain

 The Eastern Snake River Plain aquifer reaches a maximum saturated thickness of about 

4,000 feet in the central part of the plain and becomes thinner to the west (fig. 6–9). Thinning 

of the aquifer forces the deep regional groundwater flow to converge, essentially “squeezing” it 

closer to the land surface to the west. Much of this deep groundwater has nitrate at background 

concentrations, because the groundwater was recharged primarily as snowmelt in undeveloped 

areas in the eastern part of the basin. As the deep groundwater with low nitrate concentrations is 

squeezed upward, it mixes with and dilutes nitrate-contaminated groundwater in the upper part 

of the basalt aquifer. This mixture of deep and shallow groundwater, with nitrate concentrations 

above background levels, discharges to the Snake River in the Thousand Springs area.

Figure 6–9. The Eastern Snake River Plain basalt aquifer thins to the west toward the 

Thousand Springs discharge area. As groundwater flows to the southwest in Jerome 

and Gooding Counties, Idaho, deep groundwater with low nitrate concentrations moves 

upward, mixing with and diluting nitrate-rich recharge from overlying agricultural lands. 

Consequently, nitrate concentrations in groundwater discharging in the Thousand Springs 

area are relatively low—but higher than background—despite large amounts of nitrogen 

input from fertilizers and cattle manure.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 49

The upward flow of deep groundwater with low nitrate concentrations mixes with 

and dilutes the nitrate-contaminated groundwater in the upper portion of the basalt 

aquifer, where it then discharges to the Snake River in a series of large springs. 
Nitrate concentrations in groundwater sampled in the three areas of the Snake River Plain 

with mixed land uses tend to be lower than those in groundwater sampled in agricultural lands 

(fig. 6 – 8), because sources of nitrate, such as fertilizers and animal manure, are fewer and 

smaller. Of the three areas, the tributary valleys, which are located at higher elevations than the 

agricultural lands, have cooler temperatures, so agricultural production is limited to grazing 

and hay production, which supply lower amounts of nitrate to groundwater than the agricultural 

lands. The lowest nitrate concentrations of the three areas were measured in the Jackson valley 

area of the Snake River Plain study area (also known as Jackson Hole, Wyoming). In this area, 

nitrate concentrations are near background because most groundwater originates as snowmelt 

runoff from the surrounding mountains where there is very little agricultural land use.

The Eastern Snake River Plain 

aquifer discharges from the 

valley walls in the Thousand 

Springs area as numerous 

large springs (top photograph). 

The nitrate contributed by 

the spring water, combined 

with excess phosphorous 

already present in the Snake 

River, feeds excessive 

aquatic vegetation (bottom 

photograph) leading to fish 

kills in the Snake River.

Ammonia nitrogen fertilizer is 

injected into irrigation water 

in the A&B area of the Snake 

River Plain, as indicated by the 

gray discoloration of the water. 

The concrete ditch conveys 

water to the fields where it 

irrigates and fertilizes the 

crops. Excess irrigation water 

can infiltrate to groundwater, 

carrying dissolved nitrogen 

with it.

Photographs © 1995 by W.H. Mullins and published with permission Photograph by Michael G. Rupert, USGS

Ammonia

injection

point

Excessive aquatic vegetation

Large springs issuing

from valley wall

50 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 6

Nitrate concentration

6

5

4

3

2

1

0

Concentration, in milligrams

per liter as nitrogen

Background

(1 mg/L)

Mixed land use

Oahu,

groundwater

near

water table

Oahu,

public

supply

Figure 6-10.

Military land

0 4 6 8 MILES 2

0 4 6 8 KILOMETERS 2

N

EXPLANATION

Oahu public supply 

Nitrate concentration,

 in milligrams per liter

 as nitrogen

<1

1 to 2.5

>2.5

Oahu, groundwater 

 near water table

<1

1 to 2.5

> 2.5

Land use

Arrow indicates 

 generalized direction

 of groundwater flow

Agriculture

Urban

Other (mostly forest)

NAWQA groundwater 

study area

See sidebar, Boxplots, p. 41

Nitrate concentrations in Oahu groundwater were relatively low but 

substantially greater than background concentrations because of 

fertilizer application and irrigation recharge 

Groundwater samples collected on Oahu generally had lower nitrate concentrations than 

those from the Columbia Plateau or the Snake River Plain aquifers. The highest concentration 

of nitrate in groundwater was only 5.2 mg/L—more than 10 times lower than the highest 

concentration of nitrate measured in groundwater in the Columbia Plateau (59 mg/L) or the 

Snake River Plain (58 mg/L). No groundwater samples collected on Oahu had nitrate concentrations greater than the USEPA MCL, but more than half had a nitrate concentration greater 

than the background concentration of 1 mg/L (fig. 6–10), indicating that human activities have 

contributed nitrate to the groundwater. Most wells sampled on Oahu with background nitrate 

concentrations are in forested areas east and upgradient of the agricultural lands. Groundwater 

from these forested areas flows southwest beneath agricultural croplands, where large volumes 

of irrigation recharge have percolated down to the top of the water table, creating a distinct 

upper layer of groundwater that contains elevated concentrations of nitrate, pesticides, and 

other constituents. This irrigation-recharge layer is about 100 to 150 feet thick in south Oahu 

and has been studied in detail since the 1960s.(16, 17)

Figure 6 –10. Nitrate concentrations in water from wells that withdraw groundwater near the water table on Oahu 

were higher than nitrate concentrations in water from the deeper public-supply wells. The shallower wells withdraw 

groundwater from the irrigation-recharge layer, which is a distinct upper layer of groundwater near the top of the 

water table that contains elevated nitrate, pesticides, and other constituents.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 51

No groundwater samples 

collected on Oahu had 

nitrate concentrations 

greater than the USEPA 

MCL, but more than half 

had a nitrate concentration greater than the 

background concentration 

of 1 mg/L as nitrogen.
Nitrogen input, in thousands of tons

180

160

140

120

100

80

60

40

20

0

Year

A. Columbia Plateau

Nitrogen input

B. Snake River Plain

Fertilizer Dairy and beef cattle Atmospheric deposition

EXPLANATION

1940 1950 1960 1970 1980 1990 2000 2010

Year

1940 1950 1960 1970 1980 1990 2000 2010

Figure 6-11.

Nitrogen input

Nitrate concentrations in groundwater in the 

Columbia Plateau generally have stayed the 

same, but nitrate concentrations in groundwater 

in the Snake River Plain have increased in 

response to increasing agricultural sources

During 1993–2005, nitrate concentrations in Columbia 

Plateau groundwater, overall, have stayed the same.(52) The 

lack of any significant change in nitrate concentrations 

reflects the relatively constant fertilizer use in the Columbia 

Plateau since the 1980s (fig. 6–11). Groundwater samples 

were collected in three areas (the Quincy-Pasco row-crop 

area, the Quincy-Pascoe orchard area, and from the Columbia 

Plateau public-supply wells) once during 1993–95 and 

once almost 10 years later (2002). The amount of change 

in nitrate concentrations was different from one site to the 

next, but overall there was no net change—nitrate concentrations in water from about half of the wells increased and 

nitrate concentrations in water from about half of the wells 

decreased. These results are consistent with another study of 

the Columbia Plateau(53) that reported no significant change of 

nitrate concentrations in water from 474 wells sampled in the 

plateau, with the exception of wells with nitrate concentrations 

greater than 10 mg/L as nitrogen—those wells had a significant decrease in nitrate concentrations.

Nitrate concentrations in wells and springs sampled in 

some areas of the Snake River Plain were higher in 2005 than 

Figure 6–11. Nitrogen input from fertilizers in A, the Columbia Plateau and B, the Snake River Plain began to increase 

in the 1950s. In the Columbia Plateau, nitrogen input from fertilizers generally stabilized in about 1980, but in the Snake 

River Plain, nitrogen input from cattle manure and fertilizers has continued to increase through 2007. Before 1945, the 

primary sources of supplemental nitrogen for crops were natural sources such as animal manure, mineral sources 

such as potassium nitrate, and crop rotation with legume crops such as alfalfa. Inorganic nitrogen fertilizers came into 

widespread use following World War II, when facilities that had produced ammonia and synthetic nitrates for explosives 

were converted to the production of nitrogen-based fertilizers. Nitrogen input data from cattle manure and fertilizers are 

not available for Oahu.

10 years earlier. Sixty-one wells in the Jerome/Gooding and 

A&B areas of the Snake River Plain were sampled during 

1993–94 and again about 10 years later (2005). Nitrate 

concentrations in water from about two-thirds of the wells were 

higher in 2005 than in 1993–94, and nitrate concentrations in 

water from about one-third of the wells were lower. Some of 

the largest changes in nitrate concentrations occurred in the 

A&B area, where the median nitrate concentration increased by 

more than 1 mg/L as nitrogen. Nitrogen input from fertilizers 

has increased steadily in the Snake River Plain since 1950, 

and there has been a fivefold increase in the number of dairy 

cattle in a six-county region of the Snake River Plain from 

1987 to 2007 (fig. 6–12). Dairies can be a source of nitrate to 

groundwater if the waste-management controls in the area are 

ineffective.(54) Nitrogen from cattle manure used as fertilizer 

on surrounding fields also can contaminate groundwater if the 

manure is applied in excess of agronomic rates. The number 

of dairy cattle and fertilizer use in the Snake River Plain is 

projected to continue to increase and nitrate concentrations in 

groundwater are likely to increase.

52 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005

Nitrogen input from fertilizers has increased 

greatly in the Snake River Plain since 1950, and 

the number of dairy cattle in the Snake River Plain 

has increased by fivefold since 1987.
It can take more than 50 years for nitrate concentrations in groundwater from the Snake 

River Plain to respond to changes in nitrogen input at the land surface

What will the quality of Snake River Plain groundwater be in the future if nitrogen inputs continue to increase? What will it be 

if inputs stay the same? If nitrogen inputs were ceased entirely, how long would it take for nitrate concentrations in groundwater 

to return to background levels? These questions were addressed using groundwater-flow models that make it possible to project 

future nitrate concentrations on the basis of current and future agricultural practices.(51) Three scenarios were investigated: 

(1) nitrogen input continues to increase at the 

2008 rate of increase until 2028 and then levels 

off; (2) nitrogen input is fixed at 2008 levels; and 

(3) all nitrogen input from fertilizer and dairy 

manure ceases in 2008. The results are illustrated 

for a well located in the A&B area. Under the first 

scenario, nitrate concentrations in groundwater 

are predicted to increase by as much as 

2 to 4 mg/L over a period of about 50 years, 

reaching or exceeding the USEPA MCL of 

10 mg/L in some areas. Under the second 

scenario, nitrate concentrations in groundwater 

continue to increase—even though inputs do 

not increase— for about 40 years, leveling off 

at a concentration slightly less than that of scenario 1. Under the third scenario, it could take as much as 50 years for nitrate 

concentrations to return to background concentrations near 1 mg/L even if all input related to human activities were stopped in 

2008. This period of 50 years represents the “flushing time” of the aquifer. The results highlight the lag time between actions at the 

land surface and changes in the water quality of the aquifer. The lag occurs because of the time it takes for groundwater to move 

from the point of recharge at the land surface through the aquifer to a well.

Scenario 1: Nitrogen input

increases at current (2008)

rates until 2028, and 

then held constant

2000 2020 2040 2060

Scenario 2: Nitrogen input 

fixed at 2008 levels

Scenario 3: All agricultural

nitrogen input is stopped

after 2008 (precipitation

input only)

8

7

6

5

4

3

2

1

0

Concentration, in

milligrams per liter as nitrogen

Projected nitrate concentrations

in groundwater

Year

2008

2028

2048

Chapter 6

Number of dairy cattle

1920

1930

1940

1950196019701980

1990

20002010

Year

150,000

125,000

100,000

75,000

50,000

25,000

0

Gooding

Lincoln

Jerome

Minidoka

Twin Falls

Cassia

EXPLANATION

Dairy cattle

County

Figure 6-12.

Figure 6 –12. The combined number of dairy cattle in 

a six-county region of the Snake River Plain increased 

over 300,000 head between 1987 and 2007. The resulting 

increase in nitrogen input is roughly equivalent to an 

increase of 4.6 million to 6.2 million humans.

Upper: Wastewater and manure from dairies 

that are used to fertilize crops can be a source 

of nutrients to groundwater. Lower: Dairy cows 

being cooled by sprayers during the summer in 

order to maintain milk production. Nitrate-laden 

wastewater from dairies can contaminate groundwater if the wastewater is not managed properly. Photograph by USDA Photograph by Tim McCabe, USDA NRC

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 53
What is a pesticide?

A pesticide is any substance or mixture of substances intended for preventing, destroying, 

repelling, or mitigating any pest.(55) Pests can be insects, mice and other animals, unwanted plants 

(weeds), fungi, or microorganisms such as bacteria and viruses. Though often misunderstood to 

refer only to insecticides, the term pesticide also applies to herbicides, fungicides, and various 

other substances used to control pests. Most pesticides investigated in this report are herbicides 

(designed to control plants) and insecticides (designed to control insects). Generally speaking, 

insecticides are more toxic to humans and animals than are herbicides. As of 1997, about 

900 pesticides were registered in the United States for use in more than 20,000 different pesticide 

products.(56) New pesticides are introduced every year—for example, approximately 10 to 20 new 

active ingredients were registered each year from 1967 to 1997. Volatile pesticides that are used 

as soil fumigants are discussed in the Volatile Organic Compound section of this chapter.

Pesticides in the Columbia Plateau, Snake River Plain, and Oahu aquifers

In the Columbia Plateau and Snake River Plain and on Oahu, at least one 

pesticide was detected in water from about one of every two wells sampled in the three 

study areas combined. Only concentrations of diuron and dieldrin exceeded their 

human-health benchmarks, and in only a small number of wells. Patterns of detection 

reflect how pesticides are used—some broad-spectrum pesticides were detected in 

groundwater samples from all three areas, but other, more targeted pesticides were 

detected only in samples from the Columbia Plateau and Snake River Plain, or only 

in samples from Oahu. Samples commonly contained mixtures of several different 

pesticides—the health risks of mixtures of pesticides are not well known.

A crop duster sprays a sugar 

beet field with fungicide. 

The fungus Cercospora 

beticola, which causes leaf 

spot disease, is developing 

resistance to some fungicides 

currently used to control 

its growth.

Wheat harvest on the Palouse 

hills of the Columbia Plateau. 

Herbicides, such as diuron or 

metribuzin, are used to maximize 

crop yields and improve the 

purity of the harvested grain. 

Photograph by USDA ARS, Northern Plains Agricultural Research Laboratory, Sidney, Montana

Photograph by USDA ARS

54 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 6

Not analyzed* Not analyzed* *

EXPLANATION

Detected at any 

concentration 

*

(B)

(H)

(I)

Detected at a 

concentration 

≥0.1 microgram 

 per liter

Breakdown product

Herbicide

Insecticide

Hexazinone was analyzed

in Oahu samples because

of known use there, but

was not analyzed in the

Columbia Plateau or

Snake River Plain

0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50

Columbia Plateau

Pesticides detected

Snake River Plain Oahu

Percentage of all sampled wells in which constituent was detected

Atrazine (H)

Deethylatrazine (B)

Simazine (H)

Metribuzin (H)

Prometon (H)

Metolachlor (H)

Bromacil (H)

Diuron (H)

Hexazinone (H)

Dieldrin (I)

Figure 6-13.

Figure 6–13. Atrazine and its breakdown product deethylatrazine were the most commonly 

detected pesticides in groundwater samples from the Columbia Plateau, the Snake River Plain, 

and Oahu aquifers. Other pesticides detected reflect the diversity of crops grown. In groundwater 

samples from the Columbia Plateau and Snake River Plain, simazine, metribuzin, and metolachlor, 

which are used on row crops such as corn and potatoes, were detected. In groundwater samples 

from Oahu, bromacil, which is applied to pineapple crops, was the most commonly detected 

pesticide. Diuron and hexazinone, which are used on sugarcane and pineapple, also were 

detected in groundwater in Oahu.

Pesticides were detected in half of the groundwater samples in the 

Columbia Plateau and Snake River Plain and on Oahu, but rarely at 

concentrations of concern for human health

Pesticides were detected in water from about 50 percent of wells sampled in the Columbia 

Plateau, the Snake River Plain, and Oahu aquifers, but only diuron and dieldrin were measured 

at concentrations that exceeded their human-health benchmarks. Diuron exceeded its benchmark of 2.0 μg/L in water from only one well (less than 1 percent of wells sampled), and 

dieldrin exceeded its HBSL of 0.002 μg/L in water from about 2 percent of wells sampled.

Ten pesticide compounds were detected in groundwater from at least 5 percent of the wells 

sampled in at least one of the three study areas (fig. 6–13). Of those 10 compounds, 8 are herbicides, 1 is an insecticide, and 1 is a breakdown product of one of the herbicides. Atrazine and its 

breakdown product deethylatrazine were the most commonly detected pesticides in groundwater 

from the Columbia Plateau and the Snake River Plain, and the third and second most frequently 

detected pesticides on Oahu. Bromacil was the most commonly detected pesticide in groundwater on Oahu, but was much less commonly detected in the other two study areas.

Groundwater sampled in agricultural lands of the Snake River Plain had a greater occurrence of pesticides than groundwater sampled in areas of mixed land uses in the Snake River 

Plain, because pesticide use is much greater on agricultural lands than areas of mixed land 

uses. Many of the wells sampled in areas of mixed land uses in the Snake River Plain are 

located in the tributary valleys and Jackson valley where much of the land is pastureland and 

rangeland, and pesticide use is much lower than in agricultural lands. There was little difference 

in pesticide occurrence in groundwater in agricultural lands and areas of mixed land uses of the 

Columbia Plateau, because the percentage of agricultural lands is greater in the areas of mixed 

land uses of the Columbia Plateau than in the Snake River Plain.

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 55

Pesticides detected 

in groundwater of the 

Columbia Plateau, 

Snake River Plain, and 

Oahu aquifers reflect 

the different crops 

grown and pesticides 

used in those areas.
Columbia Plateau Snake River Plain Oahu

Percentage of all sampled wells with given number of detections

Number of pesticides detected 

in water from each well

1

2

3

4

5

6

7

8

9

10

Mixtures of pesticides detected 

Figure 6-14.

0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50

Some pesticides were detected in groundwater samples from all three areas, but other 

pesticides were detected only in samples from the Columbia Plateau and Snake River Plain, 

or only in samples from Oahu. This difference is because some pesticides are broad-spectrum 

pesticides that are used on many crops in different areas of the United States, whereas other 

pesticides are used on specific crops. Simazine, metribuzin, metolachlor, and prometon were 

detected in groundwater from the Columbia Plateau and the Snake River Plain, but were not 

detected in groundwater from Oahu. These pesticides are used on the row crops grown in the 

Columbia Plateau and Snake River Plain, such as potatoes, barley, and alfalfa, but not on any 

major crops grown on Oahu (such as pineapple and sugarcane).(57, 58) Bromacil, diuron, and 

hexazinone were detected frequently in groundwater from Oahu because they were used for 

weed control on sugarcane and pineapple crops (historically the major cash crops on Oahu). 

Broad-spectrum herbicides, such as atrazine, that are used on many crops in many different 

areas of the United States were detected frequently in groundwater from all three study areas.

Mixtures of pesticides commonly are detected in groundwater, and the 

health effects of these mixtures are unknown 

Water samples from about 35 percent of all wells sampled in the three study areas 

contained two or more pesticides (fig. 6–14). Results are similar on a national basis: 47 percent 

of wells sampled in agricultural lands across the United States had detections of two or more 

pesticide compounds, and about 20 percent of wells in areas of mixed land uses had detections 

of two or more pesticide compounds.(59) Toxicologists have evaluated the health risks and 

established drinking-water regulations for many individual chemicals, but risks associated 

with mixtures of chemicals are far less known and, in some cases, may be greater than those 

of individual chemicals.(60, 61)

Mixtures of pesticides in groundwater may occur because combinations of pesticides are 

more effective at controlling weeds, and thus the practice of combining pesticides is widely 

used. For example, simazine and metribuzin can be combined with atrazine for more effective 

weed control in row crops.(62) In other cases, one pesticide may be applied one year, and a 

different pesticide may be applied the following year. In areas of mixed land use, different 

pesticides may be applied to adjacent agricultural and urban lands, thus creating mixtures of 

pesticide compounds in the groundwater that were not applied together at the land surface.

Just as individual pesticides detected in groundwater can reflect the crops on which they 

were applied (and therefore the region), so too can pesticide mixtures. For example, simazine 

and metolachlor are commonly used in combination with atrazine, but not on crops grown on 

Oahu.(63) Consequently, atrazine and simazine, and atrazine and metolachlor, were two of the 

most common pesticide mixtures in groundwater samples from the Columbia Plateau and the 

Figure 6 –14. Pesticides most commonly occur in groundwater as mixtures—detections 

of two or more pesticides in a sample were more common than detection of just one 

pesticide. It is unknown what the health effects might be from drinking water with mixtures 

of pesticides, because health-based standards are based on individual compounds.

56 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 6

All three areas

Columbia Plateau and

Snake River Plain

Oahu

EXPLANATION

Location where pesticide 

 mixture was detected

Location and type of pesticide 

mixture detected

Figure 6-15.

Percentage of wells

0 10 20 30 40

Atrazine and deethylatrazine

Atrazine, deethylatrazine, and simazine

Atrazine and simazine

Atrazine, deethylatrazine, bromacil, and diuron

Atrazine, deethylatrazine, and diuron

Atrazine, deethylatrazine, and metolachlor

Atrazine, deethylatrazine, and metribuzin

Atrazine, deethylatrazine, and prometon

Snake River Plain aquifers, but these mixtures were not detected in groundwater samples from 

Oahu (fig. 6–15). As another example, atrazine, bromacil, and diuron are all applied to pineapple,(64, 65) and the mixture of atrazine (and its breakdown product deethylatrazine), bromacil, 

and diuron was the fourth most commonly detected mixture, but this mixture was detected only 

on Oahu. These observations indicate that certain mixtures could be anticipated by scientists, 

depending on the regions they are studying. Once information on the toxicity of mixtures is 

better known, then the potential health effects can be better estimated.

Trace concentrations of atrazine and its breakdown product deethylatrazine 

in groundwater appear to be an unavoidable consequence of atrazine use

Atrazine and deethylatrazine are the most commonly detected pesticides in groundwater 

across the United States,(59) including groundwater from the Columbia Plateau and Snake River 

Plain aquifers (see appendix 3); atrazine and deethylatrazine were the second most commonly 

detected pesticides on Oahu (fig. 6–13). Atrazine has been widely detected in groundwater 

in Europe, where it was banned for use in all of the European Union member states in 2005 

because of concerns for groundwater contamination.(66) Swiss-based Syngenta, the company 

that manufactures atrazine, agreed to set up a $105 million fund in 2012 to cover the costs of 

removing the herbicide from drinking water for as many as 2,000 water suppliers in six States 

of the United States.(67) Statistical correlations show that atrazine is detected most frequently 

in areas with a high percentage of land in row crops on which atrazine is used (such as corn or 

sugarcane) and highly permeable soils with low amounts of organic matter.(68)

Figure 6–15. The types of crops grown in an area determine the mixtures of pesticides 

that are likely to be detected in groundwater. For example, the mixture of atrazine, 

deethylatrazine, and simazine was detected in groundwater from the Columbia Plateau 

and the Snake River Plain, but not in groundwater from Oahu, because simazine is not 

used on crops grown on Oahu. Atrazine and its breakdown product deethylatrazine

 were detected in all three areas because atrazine is widely used in all of them.

Sugarcane being 

harvested into high 

dump trailers for 

transport to a sugar 

mill. Atrazine was used 

on sugarcane on Oahu 

until the sugarcane 

industry was phased 

out in 1996.

Photograph by David Nance, USDA ARS

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 57
Although dieldrin and its parent compound, aldrin, were banned about 

16 years prior to sample collection, dieldrin is still detected in groundwater

Dieldrin was detected in groundwater above the low HBSL of 0.002 μg/L in water from 

3 of the 44 wells sampled on Oahu (almost 7 percent of the wells) (fig. 6–13). The HBSL for 

dieldrin is a range of values; the low value (0.002 μg/L) corresponds to a one in one million 

cancer risk, the high value (0.2 μg/L) corresponds to a one in ten thousand cancer risk. The 

range of HBSLs is set at these very low concentrations because dieldrin is very toxic to humans.

Historically, dieldrin was used as an insecticide, but dieldrin is also a breakdown product 

of the insecticide aldrin, which rapidly breaks down to dieldrin once in the environment. Aldrin 

was used heavily as a termiticide in urban areas on Oahu prior to 1984, although dieldrin itself 

was not commonly used.(69) Even though aldrin has not been used on Oahu since 1984, its 

breakdown product dieldrin is still detected in groundwater. Aldrin and dieldrin are organochlorine pesticides, which in the past were used heavily in agriculture and to control termites 

and mosquitoes. DDT (dichloro-diphenyl-trichloroethane) also is an organochlorine pesticide. 

In the United States, the use of most organochlorine pesticides was discontinued in the 1970s 

for agriculture, and in the 1980s for termite control, because these pesticides do not readily 

break down in the environment and because concentrations tend to increase in animal tissue 

as they are passed up the food chain.(70) Dieldrin was detected more frequently in groundwater 

samples from Oahu than from the Columbia Plateau or the Snake River Plain aquifers, possibly 

because Oahu is more urbanized than the Columbia Plateau and the Snake River Plain and 

because termite damage is a greater problem in the tropical climate of Oahu.

Termites cause more than $100 million in damage each year to wooden 

structures on Hawaii. The Formosan subterranean termite shown here 

is one of the most destructive of the seven termite varieties found on

Hawaii.(90) Aldrin was an effective control for termites, but has not been 

used on Oahu since 1984 because of environmental concerns. Even 

though aldrin has not been used on Oahu since 1984, dieldrin (a break­down product of aldrin) is still being detected in Oahu groundwater.

Photograph by Scott Bauer, USDA ARS

Photograph by Scott Bauer, USDA ARS

Sugarcane production on Oahu used atrazine 

for weed control. Even though sugarcane has 

not been grown on Oahu since 1996, atrazine 

is still detected in Oahu groundwater. 

58 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005

Dieldrin is very toxic 

to humans, so the 

HBSL was set at a very 

low concentration.
Chapter 6

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 59

(S)

(F)

(D)

Columbia Plateau Snake River Plain Oahu

Percentage of wells in which compound was detected

1,2,3-trichloropropane (TCP) (F)

Trichloroethylene (TCE) (S)

1,2-dichloropropane (DCP) (F)

Carbon tetrachloride (S)

Perchloroethene (PCE) (S)

Chloroform (D,S)

1,2,4-trimethylbenzene (S)

Ethylene dibromide (EDB) (F)

cis-1,2-dichloroethene

(cis-1,2-DCE) (S)

EXPLANATION

Detected at a 

concentration

≥0.2 microgram

per liter

Solvent

Fumigant

Disinfection 

byproduct

0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50

VOCs detected

Figure 6-16.

Volatile Organic Compounds (VOCs) in the Columbia Plateau, Snake River 

Plain, and Oahu aquifers

Solvents and soil fumigants were the VOCs most 

frequently detected in groundwater from the Columbia 

Plateau, Snake River Plain, and Oahu aquifers. VOCs 

were detected much more commonly on Oahu than in the 

Columbia Plateau and Snake River Plain. For example, 

more than half of the wells sampled on Oahu contained 

at least one VOC, whereas in the Snake River Plain, only 

2 percent of the wells contained at least one VOC. The 

Columbia Plateau, the Snake River Plain, and Oahu 

are among the few areas in the United States where soil 

fumigants have been detected in groundwater at concentrations above human-health benchmarks. Although 

several soil fumigants were banned from use during the 

late 1970s, they are still being detected in groundwater 

decades later.

VOCs were commonly detected in groundwater samples from Oahu, less 

frequently in the Columbia Plateau, and rarely in the Snake River Plain 

Volatile organic compounds were far more commonly detected in groundwater samples from 

Oahu than in groundwater samples from the Columbia Plateau or Snake River Plain (fig. 6–16). 

Water from more than half of the wells sampled on Oahu contained at least one VOC, and water 

from one well contained four VOCs in the same sample.(68) In comparison, water from only 

2 percent of the wells sampled in the Snake River Plain contained at least one VOC.

More solvents were detected than soil fumigants; of the nine VOCs detected in 

groundwater at or above the laboratory reporting level of 0.2 μg/L, six were solvents 

and three were soil fumigants. The most commonly detected VOC was the soil fumigant 

1,2,3-trichloropropane (TCP), followed by the solvent trichloroethylene (TCE) and the soil 

fumigant 1,2-dichloropropane (DCP).

What are volatile organic compounds?

Volatile organic compounds make up a broad class of 

organic compounds that volatilize (evaporate) readily, and 

most are soluble in water. Many everyday chemicals are VOCs, 

including solvents and degreasers, refrigerants, soil fumigants, 

gasoline, and chlorination disinfection byproducts. Many VOCs 

have multiple uses—for example, some solvents also are used 

as additives in pesticide formulations. Groundwater can be 

contaminated by VOCs in a number of ways: accidental spills, 

improper disposal, leakage from point sources (such as storage 

tanks), and widespread application of chemical products to the 

land surface (such as for agriculture) are just some examples. 

VOCs can even dissolve into water from the atmosphere, where 

they are present at low concentrations from industrial and 

manufacturing releases.(71)

Figure 6–16. VOCs were detected more frequently in wells on Oahu than in wells in the 

Columbia Plateau or the Snake River Plain. Of the nine VOCs detected at or above the 

laboratory reporting level of 0.2 μg/L, six were solvents and three were soil fumigants.
Some high-capacity publicsupply wells on Oahu have 

been closed because soil 

fumigants were detected in 

the groundwater. 

What is a soil fumigant?

Soil fumigants are VOCs and also pesticides 

that are applied to soils to reduce populations of 

nematodes, weeds, fungal pathogens, and other 

soil-borne microorganisms.(72, 73) Soil fumigants 

are injected or incorporated into the soil before 

crops are planted; the fumigant vapor permeates 

the soil and kills soil-borne pests. Soil fumigants 

are used on many different crops across the 

Nation.(72) In the Columbia Plateau and the 

Snake River Plain, soil fumigants are used 

most commonly on potatoes and sugar beets, 

and on Oahu they are used most commonly on 

pineapple (table 6–2).

Although most of the soil fumigants 

detected in groundwater in the Columbia 

Plateau and Snake River Plain and on Oahu 

have been banned for use in the United States, 

they have been replaced by other soil fumigant 

compounds such as metam sodium, methyl 

bromide (bromomethane), and chloropicrin.(72) Although there are no chemical-use estimates of soil fumigants in the three 

study areas, during 2001, metam sodium was the third most commonly used pesticide in the United States (57 to 62 million 

pounds), methyl bromide was the seventh most commonly used pesticide (20 to 25 million pounds), and chloropicrin was the 

eighteenth most commonly used pesticide (5 to 9 million pounds).(74) To date (2012), these new soil fumigants have not been 

routinely detected in groundwater in the United States, although only methyl bromide can be analyzed by using common 

laboratory methods.

A tractor is used to inject the fumigant 1,3-dichloropropene into the 

soil at a depth of 46 inches. Following the tractor is a disk and ring 

roller, which compacts the soil and closes any fractures to lessen the 

release of the fumigant vapor to the atmosphere.

Photograph by USDA ARS, U.S. Salinity Laboratory

Soil fumigants were commonly detected in groundwater on Oahu, in 

some cases at concentrations that exceed human-health benchmarks

Soil fumigants were frequently detected in groundwater on Oahu, less frequently in 

groundwater in the Columbia Plateau, and in only one well in the Snake River Plain 

(table 6 –1). Soil fumigants historically have been the most intensively used class of pesticides 

on Hawaii. On Oahu, soil fumigants were detected in almost half—44 percent—of the wells 

sampled.(46) Concentrations of DBCP and TCP were greater than Hawaii MCLs in water from 

about 10 percent of the wells sampled on Oahu. Hawaii MCLs for some soil fumigants are 

lower than the MCLs established by the USEPA.(75)

Although soil fumigants were detected less frequently in groundwater from the Columbia 

Plateau and Snake River Plain than Oahu, a few samples contained soil fumigants at concentrations greater than human-health benchmarks. In the Columbia Plateau, two water samples 

had concentrations of EDB greater than its USEPA MCL of 0.05 μg/L, and one groundwater 

sample from the Columbia Plateau and one from the Snake River Plain had a concentration of 

DCP that was greater than its USEPA MCL of 5 μg/L.

Several of the soil fumigants detected in groundwater from the Columbia Plateau, the 

Snake River Plain, and Oahu are highly carcinogenic (cancer causing), which is reflected in 

the very low concentrations considered safe for drinking water by the USEPA and the State of 

Hawaii (fig. 6–17; table 6 –2). Detection of soil fumigants in groundwater on Oahu has resulted 

in closures of some high-capacity public-supply wells and in others has required installation 

of costly carbon filtration systems to make the water drinkable (see sidebar, History of soil 

fumigant detection and water treatment on Oahu, p. 63).

Even though most of the soil fumigants reported here have not been used since the early 

1980s, they are persistent and can be detected in groundwater for decades.(76) Thus, soil fumigants 

are considered “legacy contaminants”— current contamination originating from use decades ago.

60 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Table 6 –1. Soil fumigants were detected most frequently in wells on Oahu. Groundwater sampled on Oahu had the greatest 

number of concentrations greater than human-health benchmarks, in part because Hawaii MCLs are lower than those set by the 

USEPA. These results are for samples collected from all types of wells sampled for soil fumigants, not just the drinking-water 

wells discussed in chapter 5.

[μg/L, micrograms per liter (parts per billion); nd, not detected; USEPA, U.S. Environmental Protection Agency; USGS, U.S. Geological Survey; 

MCL, Maximum Contaminant Level; HBSL, Health-Based Screening Level; H-MCL, Hawaii MCL; MCLs and HBSLs are human-health benchmarks 

for drinking water (for regulated and unregulated compounds, respectively) and are in micrograms per liter (parts per billion)]

Fumigant

compound

Columbia Plateau Snake River Plain Oahu 

Number

of wells 

sampled

Number of

detections

Number of

detections 

above USEPA

MCL or

USGS HBSL

Number 

of wells 

sampled

Number of 

detections

Number of

detections 

above USEPA 

MCL or

USGS HBSL

Number 

of wells 

sampled

Number of 

detections

Number of

detections 

above USEPA 

MCL or

USGS HBSL

Number of

detections 

above

Hawaii

MCL

1,2-dibromo-3-

chloropropane 

(DBCP)

162 nd nd 124 nd nd 45 8 0

(USEPA 

MCL=

0.2 μg/L)

4 

(H-MCL=

0.04 μg/L)

1,2-dibromoethane (EDB, 

ethylene 

dibromide)

162 3 2 

(MCL=

0.05 μg/L)

124 nd nd 45 4 0 

(USEPA 

MCL=

0.05 μg/L)

0

(H-MCL=

0.04 μg/L)

1,2-dichloropropane

(DCP)

151 9 1 

(MCL=

5 μg/L)

128 1 1 

(MCL=

5 μg/L)

45 19 0

(USEPA

MCL =

 5 μg/L)

No H-MCL

established

1,1-dichloropropene

(DCPe)*

151 nd nd 128 nd nd 45 1 No humanhealth 

benchmark 

established

No H-MCL

established

1,2,3-trichloropropane

(TCP)

151 4 0 

(HBSL=

40 μg/L)

128 nd nd 45 18 0

(USGS 

HBSL=

40 μg/L)

6 

(H-MCL=

0.8 μg/L)

*The detected compound, 1,1-dichloropropene, is a breakdown product of 1,3-dichloropropene (DCPe) still used in fumigant formulations.

Chapter 6

1,2,3-trichloropropane (TCP)

1,1-dichloropropene (DCPe)

1,2-dichloropropane (DCP)

1,2-dibromoethane

(EDB, ethylene dibromide)

1,2-dibromo-3-

chloropropane (DBCP)

Hawaii

MCL=0.8

USEPA

MCL=5

Concentration, in micrograms per liter

0.001 0.01 0.1 1 10 100

 HBSL=40 Columbia Plateau

Snake River Plain

Oahu

EXPLANATION

USEPA

MCL=0.2

USEPA

MCL=0.05

Figure 6-17.

Soil fumigants detected

Hawaii

MCL=0.04

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 61

Figure 6–17. Soil fumigants were detected 

in groundwater samples from all three study 

areas, but most frequently in samples from 

wells on Oahu. Several wells on Oahu contained 

one or more soil fumigants at a concentration 

exceeding the Hawaii MCL, which are more 

stringent than the USEPA MCLs. There are no 

USEPA MCLs for DCPe or TCP and no Hawaii 

MCLs for DCP or DCPe.
Table 6–2. Soil fumigants detected in groundwater in the Columbia Plateau, Snake River Plain, and Oahu aquifers have long

histories of use, dating back to the 1940s and 1950s. Only 1,3-dichloropropene is still used as a soil fumigant in the United States;

the other compounds have been banned for soil application because of environmental concerns. Hawaii MCLs are lower than 

those set by the USEPA. 

[USEPA, U.S. Environmental Protection Agency; MCL, Maximum Contaminant Level; USGS, U.S. Geological Survey; HBSL, Health-Based

Screening Level; MCLs and HBSLs are human-health benchmarks for drinking water (for regulated and unregulated compounds, respectively) 

and are in micrograms per liter (parts per billion)]

Fumigant compound

Years of use as a

soil fumigant in the 

three study areas

Used principally

for these crops

Human-health benchmarks

(micrograms per liter)

USEPA

MCL 

Hawaii

MCL 

USGS

HBSL 

1,2-dibromo-3-chloropropane (DBCP) 1955–1977 Pineapple, potatoes, 

sugar beets, orchards

0.2 0.04 None

1,2-dibromoethane (EDB, 

ethylene dibromide)

1948–1983 Pineapple, potatoes 0.05 0.04 None

1,2-dichloropropane (DCP) 1943–1977 Pineapple, potatoes 5 None None

1,3-dichloropropene (DCPe)* 1956–

present (2012)

Pineapple, potatoes, 

sugar beets, carrots

None None None

1,2,3-trichloropropane (TCP) 1943–1977 Pineapple, potatoes None 0.8 40

*The detected compound, 1,1-dichloropropene, is a breakdown product of 1,3-dichloropropene (DCPe) still used in fumigant formulations.

Scientists examine a sugar beet 

damaged by root-knot nematodes. 

Root-knot nematodes cause 

extensive damage to a wide 

variety of economically important 

crops, including sugar beets, by 

reducing the yields and quality 

of the crops. The soil fumigant 

1,3-dichloropropene is an effective 

chemical control against the rootknot nematode.

Photograph by Scott Bauer, USDA ARS

62 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
History of soil fumigant detection and water treatment on Oahu

Soil fumigants have been detected in Oahu groundwater since 1980.(77–79) Detection of soil fumigants by interagency 

sampling led by the Hawaii Department of Health resulted in temporary closures of public-supply wells during the 1980s. 

Ten high-capacity public-supply wells had been ordered closed by 1983, with each closure resulting in front-page newspaper 

headlines, great concern in the affected communities, and substantial challenges to public agencies tasked with addressing 

the problem. Eventually, costly granular activated carbon filters were installed at affected wells to treat pumped groundwater 

to levels meeting drinking-water standards so that public use of the water could resume. Carbon filtration still is being used 

today with associated operating costs to maintain the filter apparatus and replace the carbon when it reaches its sorption 

limit. In 2002, the Honolulu Board of Water Supply (the operator of the affected public wells) was awarded nearly $20 million 

from three chemical manufacturers and three Oahu pineapple growers as compensation for the contaminated 

groundwater source.(80)

These large vessels contain granular 

activated carbon, which is used to 

remove soil fumigants from drinking 

water at a Honolulu Board of Water 

Supply well field in central Oahu. 

Prior to discovery of soil fumigants 

in the 1980s, water from Oahu’s 

volcanic-rock aquifers typically 

required no treatment or only slight 

chlorination before use.

Photograph courtesy of GMP International

Photograph by Stephen S. Anthony, USGS

Chapter 6

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 63

Because VOCs in air can readily 

dissolve into water, groundwater 

samples for VOCs are collected 

from Teflon tubing within a bagged 

enclosure to reduce the possibility 

of contamination. The tubing is 

directly connected at its far end to 

the wellhead (out of view) so that 

the water sample does not come 

in contact with the air until it is 

placed in the sample bottle. For 

more information on groundwater 

sampling methods used by the 

USGS, see the USGS National 

Field Manual.(91)
On Oahu, VOCs commonly occur in groundwater that also contains 

pesticides and nitrate

Groundwater samples on Oahu commonly contain mixtures of solvents, soil fumigants, 

pesticides, and elevated nitrate concentrations (fig. 6–18), indicating that water from many 

wells contains contaminants originating from several sources or activities at the land surface. 

Some samples contained as many as four VOCs, as well as trace concentrations of pesticides 

and elevated concentrations of nitrate.

Specific sources of the solvent contamination in groundwater on Oahu have not 

been determined despite a decade of investigations.(81) The most likely sources of solvent 

contamination on Oahu include the use of solvents at military bases, aircraft and automotive 

maintenance shops, painting shops, drum-disposal and burn areas, and dry cleaners.(46, 82)

Solvents were used intensively at military installations in central Oahu as early as the 1940s.

Soil fumigants in groundwater on Oahu are attributable mainly to pineapple cultivation. 

The pesticides in groundwater on Oahu originated from agricultural and urban use. The largest 

source of elevated nitrate concentrations in groundwater on Oahu is agricultural activities, 

primarily pineapple and sugarcane cultivation.

Chloroform, a VOC, generally was detected in groundwater samples from Oahu that also 

contained other solvents, soil fumigants, or pesticides. The chloroform might have contaminated Oahu’s groundwater as a mixture with other VOCs, as a carrier agent in pesticides or soil 

fumigants, or as a breakdown product of carbon tetrachloride.(46) The chlorination of drinking 

water, a common source of chloroform in many settings, is not a likely source on Oahu because 

most wells are located where lawn and landscape irrigation are not likely to be important 

sources of recharge to groundwater.(46)

Groundwater in Honolulu is 

free of most contaminants 

because it receives recharge 

from a mountainous area with 

no major contaminant sources. 

Groundwater in Honolulu flows 

upward, which inhibits transport 

of contaminants from land 

surface down into the aquifer. 

These factors combine to 

produce a well-protected urban 

groundwater resource.

Pineapple cultivation in Hawaii 

uses soil fumigants to control 

damaging nematodes. Soil 

fumigants historically have been 

the most intensively used class 

of pesticides in Hawaii, with 

2,200 tons applied statewide 

in 1977 compared to 1,400 tons 

of herbicides. Many of these 

soil fumigants have been 

discontinued or banned. 

Photograph by Michael G. Rupert, USGS Photograph by Douglas Peebles, 

Douglas Peebles Photography

64 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Chapter 6

Chapter 6: Understanding Where and Why Key Contaminants Occur in Groundwater 65

Military

land

Groundwater from

pristine mountain

recharge

Groundwater with

elevated nitrate

concentrations and 

fumigants from

agricultural sources

Groundwater with

elevated solvents

from military sources

Honolulu urban core

0 5 10 MILES

0 5 10 KILOMETERS

Land use

General direction

 of groundwater flow

Agriculture

Urban

Other (mostly 

forest)

NAWQA groundwater 

study area

Solvents Fumigants

Nitrate Pesticides

Organic compound classes in groundwater

Public-supply well

Monitoring well

Concentrations, in micrograms per liter

Not detected (minimum reporting level varied by compound)

Low <0.1

Medium 0.1 to 1

High >1

Natural background 0.1 to 1

Low >1 to 2

Medium >2 to 3

High >3 to 5.5

Figure 6-18.

EXPLANATION

Solvents, fumigants, and pesticides

Nitrate

Figure 6–18. Mixtures of solvents, soil fumigants, pesticides, and elevated nitrate concentrations were 

common in groundwater samples on Oahu. Many samples from central Oahu contained moderate to high 

concentrations in several of those chemical classes, reflecting inputs from agricultural, military, and urban 

sources. Concentrations were low, or not detected, in water from wells in the Honolulu urban core because 

most of the groundwater pumped from these wells originates as recharge in the undeveloped mountains to 

the northeast. The color-coded squares in this figure represent the maximum concentration in each of three 

chemical classes (solvents, soil fumigants, and pesticides) and nitrate concentration. Blue colors indicate 

that no contamination was detected; warmer colors indicate increasingly higher concentrations.
Pineapple is an important row crop grown on Oahu.

Photograph by Douglas Peebles, Douglas Peebles Photography
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View of the Grand Teton, Jackson valley study area.

72 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Glossary A

agronomic rate An application rate of 

nutrients that matches the annual require-

ments of a specific crop. Nutrients (including 

nitrogen and phosphorous) applied to a crop 

in excess of the agronomic rate can leach 

down to the groundwater. 

alluvial aquifer An aquifer composed of 

unconsolidated material, such as sand and 

gravel, deposited by a river or other 

flowing water.

anoxic Water with no dissolved oxygen 

or a very low concentration (less than 

0.5 milligram per liter) of dissolved oxygen.

apparent groundwater age The time 

elapsed since the recharge water became 

isolated from the atmosphere. The term 

“age” is normally qualified with the word 

“apparent” to signify that the accuracy of the 

determined age depends on many variables.

aquifer A geologic formation, group 

of formations, or part of a formation that 

contains a sufficient amount of saturated 

permeable material (for example, soil, sand, 

gravel and (or) rock) to yield significant 

quantities of water to wells and springs. B

background concentration A concentration 

of a substance in a particular environment that 

corresponds to minimal influence by human 

(anthropogenic) sources or activities.

bedrock General term for consolidated 

(solid) rock that underlies soils or other 

unconsolidated material. 

breakdown product A compound derived 

by chemical, biological, or physical action 

upon a pesticide. The breakdown is a natural 

process that may result in a more toxic or less 

toxic compound and a more persistent or less 

persistent compound.

C

common assessment level A single concen-

tration threshold used to establish an equal basis 

for comparing detection frequencies among 

multiple chemicals. Use of a common assess-

ment level avoids biases in detection frequencies 

caused by one compound having a lower 

detection level than another. Also sometimes 

referred to as a “common detection level.”

confining layer Geologic material with little 

or no permeability or hydraulic conductivity. 

Water does not pass through this layer or the 

rate of movement is extremely slow.

constituent A chemical or biological 

substance in water, sediment, or biota that can be 

measured by an analytical (laboratory) method.

contaminant For the purposes of this report, 

any manmade compound at any concen-

tration, or any constituent with a geologic 

source measured at a concentration exceeding 

the designated human-health benchmark.

crystalline rocks Igneous or metamorphic 

rocks consisting wholly of crystals or frag-

ments of crystals. Granite and schist are 

examples of crystalline rocks. D

denitrification The bacterial reduction of 

dissolved nitrate to nitrogen gas. Denitrification is the primary process by which nitrate 

can be eliminated naturally in groundwater.

discharge The rate of flow of surface 

water or groundwater past a given point at a 

given moment, expressed as volume per unit 

of time. Also, the outflow from an aquifer, 

spring, or well or up through a streambed.

dissolution The process of dissolving a 

solid (mineral) into a homogeneous solution 

(water). Dissolution reactions result in the 

addition of ions to water as minerals react 

with water. Common dissolution reactions 

include dissolution of carbonate rock 

(limestone or dolomite) and incongruent 

dissolution of silicate minerals (feldspar) 

by carbonic acid (H

2

CO

3

). 

Glossary

Glossary 73
domestic well A privately owned well 

that typically serves one home and supplies 

water for human consumption and other 

homeowner uses.

drinking-water standard or guideline A 

threshold concentration in a public drinkingwater supply designed to protect human 

health or to identify acceptable concentrations 

of constituents that cause unpleasant taste, 

odor, or color in the water.

E

eutrophication The enrichment of water by 

nutrients, most commonly phosphorus and 

nitrogen. During eutrophication, respiration 

processes that use organic matter cause a 

marked decline in dissolved-oxygen concentrations of water.

F

flow path The route or pathway of water 

flowing through the hydrologic system. Typically refers to subsurface (groundwater) flow.

fumigant A pesticide in the volatile organic 

compound (VOC) chemical class that is 

applied to soils to reduce populations of 

plant parasitic nematodes (harmful rootworms), weeds, fungal pathogens, and other 

soil-borne microorganisms.

G

groundwater Water that exists beneath the land 

surface, but most commonly refers to water in 

fully saturated soils and geologic formations.

groundwater age See apparent groundwater age.

groundwater discharge The flow of water 

from the saturated zone, for example, from a 

spring or a well or as seepage to surface water.

groundwater flow path See flow path.

groundwater recharge The infiltration 

of water to the saturated zone. Also refers 

to water that reaches the water table by 

infiltration of precipitation or irrigation water 

through the unsaturated zone or by seepage 

of water from surface-water bodies, such as 

streams and lakes.

groundwater sustainability The amount of 

groundwater that will be available to support 

future uses of a particular aquifer or groundwater resource. The development and use of 

groundwater in a manner that can be maintained for an indefinite time without causing 

unacceptable environmental, economic, or 

social consequences.

H

half-life The time required for the concentration of a compound in a given environmental medium to be reduced to half of its 

original value by one or more processes, 

such as breakdown or transport into another 

environmental medium.

Health-Based Screening Level (HBSL) An 

estimate of concentration (for a noncarcinogen) 

or concentration range (for a carcinogen) in 

water that (1) may be of potential humanhealth concern, (2) can be used as a threshold 

value against which measured concentrations 

of contaminants in ambient groundwater 

samples can be compared, and (3) is consistent with U.S. Environmental Protection 

Agency Office of Water methodologies.

herbicide A chemical pesticide designed to 

control or destroy plants, weeds, or grasses.

human-health benchmark A threshold 

concentration above which the concentration 

of a contaminant in drinking water could have 

adverse effects on human health. Treatment or 

other measures can be used before the water 

is consumed to lower the concentration of the 

contaminant below the benchmark.

hydraulic conductivity The volume of 

water that will move in a porous medium in a 

given amount of time under a given hydraulic 

gradient through a given area measured 

at right angles to the direction of flow. A 

measure of the ease with which a fluid moves 

through a porous or fractured material.

hydrogeologic setting A unit with common 

hydrogeologic characteristics and therefore 

common susceptibility to contamination; 

a composite description of all the major 

geologic and hydrologic factors that affect and 

control the movement of groundwater into, 

through, and out of an area.

74 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
hydrologic system The assemblage of pathways

by which water travels as it circulates beneath, 

at, and above the Earth’s surface during 

precipitation, runoff, evaporation, infiltration, 

transpiration, and groundwater discharge. I

infiltration Movement of water, typically 

downward, into soil or porous rock.

intrinsic susceptibility A measure of the 

ease with which a contaminant in water enters 

and moves through an aquifer; a characteristic 

of the aquifer and overlying material and 

hydrologic conditions independent of the 

chemical characteristics of the contaminant 

and its sources.

irrigation recharge layer A layer of poorquality groundwater (resulting from irrigation 

return flow) that overlies better-quality 

groundwater recharged under background 

conditions from distant sources. 

irrigation return flow The part of irrigation 

water applied to the surface that is not taken 

up by plants or lost by evaporation and that 

migrates to an aquifer or surface-water body. L

land-use study A study by the U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program to assess the 

effects of a specific land-use type (generally 

agricultural or urban) on groundwater quality, 

in most cases by sampling groundwater from 

monitoring wells that tap water from or near 

the water table.

leaching The removal of materials in solu-

tion from soil or rock to groundwater; refers 

to movement of pesticides or nutrients from 

land surface to groundwater. 

legume A member of the large plant family

Leguminosae, many of which harbor nitrogenfixing bacteria on their roots, and many of 

which contain edible parts. Beans, alfalfa, 

and mesquite are examples of legumes. M

major aquifer A regionally extensive 

subsurface geologic formation or group of

formations that is used, or has the potential to 

be used, as a substantial groundwater resource.

major aquifer study A study by the U.S. 

Geological Survey National Water-Quality 

Assessment (NAWQA) Program that involves 

sampling of water at 20 to 30 domestic and 

(or) public-supply wells that withdraw water 

from major aquifers. The major aquifer 

studies represent a mix of land uses and target 

water that is used for drinking-water supply. 

Maximum Contaminant Level (MCL)

Maximum permissible level of a contaminant 

in water that is delivered to any user of a 

public water system. MCLs are enforceable 

standards established by the U.S. Environ-

mental Protection Agency. 

methemoglobinemia A health condition 

characterized by reduced ability of the blood 

to carry oxygen. Infants are most affected. One

of the most common causes is nitrate in drinking

water. Also called “blue baby syndrome.”

micrograms per liter (μg/L) A unit 

expressing the concentration of a chemical 

constituent as weight (micrograms) of 

constituent per unit volume (liter) of water; 

equivalent to one part per billion in most 

streamwater and groundwater. 

milligrams per liter (mg/L) A unit expressing 

the concentration of a chemical constituent 

as weight (milligrams) of constituent per 

unit volume (liter) of water; equivalent to 

one part per million in most streamwater and 

groundwater. One thousand micrograms per 

liter (μg/L) is equivalent to 1 mg/L. 

monitoring well A well used to measure 

water quality or groundwater levels continu-

ously or periodically. Not typically used as a 

source of drinking water. Sometimes referred 

to as an “observation well.” N

nitrate An ion consisting of one nitrogen 

atom and three oxygen atoms (NO

3

– ). Nitrate 

is a plant nutrient and is very mobile in soils. 

nonpoint source A contaminant source that 

is not a discrete point, such as a pipe, ditch, 

or tunnel. Areas of fertilizer and pesticide 

applications, atmospheric deposition, and 

stormwater runoff are examples of sources

of nonpoint contamination.

Glossary

Glossary 75
nutrient An element or compound essential 

for animal and plant growth. Common 

nutrients include nitrogen, phosphorus, and 

potassium, such as are found in fertilizer.

O

organic carbon Carbon that originates 

from plants or animals and is bound in an 

organic compound.

organic matter Matter resulting from the 

decay of a plant or an animal and containing 

organic carbon compounds. Organic matter is 

rich in nutrients and is an essential component 

of soils.

oxic Water with a concentration of 

dissolved oxygen greater than or equal to 

0.5 milligram per liter.

oxidation The loss of electrons by a 

chemical species as a result of transfer to 

another chemical species, typically dissolved 

oxygen. The species donating electrons 

is “oxidized.”

P

perched groundwater Unconfined groundwater separated from an underlying body of 

groundwater by an unsaturated zone.

permeability A measure of the relative ease 

with which a porous or fractured medium can

transmit groundwater. Rock formations that

transmit fluids readily are described as permeable.

permeable Capable of transmitting liquids 

or gases through pores or openings.

pesticide Any substance, organic or 

inorganic, used to kill plant or animal pests.

point source A stationary location or 

fixed facility from which contaminants are 

discharged, for example, a pipe, ditch, ship, 

ore pit, or factory smokestack.

precipitation Any or all forms of water 

particles that fall from the atmosphere, such 

as rain, snow, hail, and sleet. Also, the process 

in which a solid is formed from a fluid 

supersaturated with dissolved ions.

Principal Aquifer A regionally extensive 

aquifer or aquifer system that has the potential 

to be used as a source of potable water. A 

Principal Aquifer can be composed of one or 

more major aquifers.

public-supply well A privately or publicly 

owned well that provides water for public use 

to (1) community water systems, (2) transient 

noncommunity water systems, such as 

campgrounds, or (3) nontransient, noncommunity systems, such as schools.

R

radioactive decay The spontaneous emission of particles (alpha or beta) and gamma 

rays from an atom with an unstable nucleus 

(radionuclide).

recharge The addition of water to the 

saturated zone naturally by precipitation 

or runoff or artificially by spreading or 

injection. Also, the water that is added.

reduction The gain of electrons by a 

chemical species, usually dissolved oxygen, 

as a result of transfer from another chemical 

species. The species accepting electrons is 

“reduced.” Once all of the dissolved oxygen 

has been reduced, other chemical species 

can accept electrons, following the most 

energetically favorable order.

reduction-oxidation (redox) Chemical 

reactions that involve the transfer of electrons 

from one chemical species to another, 

resulting in a change in the valence state of 

the species. Redox processes in groundwater 

often are microbially facilitated.

runoff Excess rainwater or snowmelt that 

is transported to streams by flow over the 

land surface.

S

saprolite A soft, earthy, thoroughly decomposed rock, typically clay-rich, occurring 

in the lowest zone of the soil profile and 

resulting from the deep weathering of the 

bedrock surface.

76 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
saturated The condition in which all the 

pores (voids, interstices) within a material are 

filled with a liquid, typically water.

saturated zone The region in the subsurface 

in which all the spaces (pores and fractures) 

are filled with water and are under pressure 

greater than atmospheric pressure.

Secondary Maximum Contaminant Level 

(SMCL) Guidelines set by the U.S. Environ-

mental Protection Agency for concentrations 

of “nuisance” constituents in drinking water 

that may cause unwanted effects, such as 

unpleasant taste, color, or odor; discoloration 

of skin or teeth; or corrosion or staining of 

plumbing fixtures. Public drinking-water 

systems are recommended but not required 

to comply with these guidelines.

sedimentary rocks Rocks composed of 

particles derived from the erosion or weath-

ering of preexisting rocks. Sandstone and 

limestone are examples of sedimentary rocks.

sorption The general process by which 

solutes, ions, and colloids become attached 

to solid matter.

Study Unit A major hydrologic system of 

the United States, geographically defined by 

surface- or groundwater features, in which 

U.S. Geological Survey National WaterQuality Assessment (NAWQA) Program 

sampling studies are focused.

susceptibility See intrinsic susceptibility. T

trace element An element found in only 

minor amounts (concentrations less than 

1 milligram per liter) in water or sediment; 

includes arsenic, cadmium, chromium, 

copper, lead, mercury, nickel, and zinc.

U

unconfined aquifer An aquifer that has 

a water table; an aquifer containing uncon-

fined groundwater.

unconsolidated deposit Deposit of loosely 

bound sediment that typically fills topographi-

cally low areas. 

unsaturated zone A subsurface zone 

containing both water and air. The unsaturated 

zone is limited above by the land surface and 

below by the water table. V

volatile organic compound (VOC) An 

organic chemical that has a high vapor 

pressure relative to its water solubility. VOCs 

include components of gasoline, fuel oils, 

lubricants, organic solvents, fumigants, some 

inert ingredients in pesticides, and some 

byproducts of chlorine disinfection.

vulnerability The tendency or likelihood for 

contaminants to reach a specified position in 

the groundwater system after introduction at 

some location above the uppermost aquifer. 

The vulnerability of a groundwater resource 

to contamination depends both on the intrinsic 

susceptibility of the resource and on the 

locations and types of human and geologic 

sources of contaminants, locations of wells, 

and characteristics of the contaminant(s). W

water table The upper surface of the 

saturated zone below which all voids (spaces) 

are filled with water. Y

young groundwater Groundwater that was 

recharged since the mid-1950s, as indicated 

by tritium concentrations in groundwater 

greater than 0.5 tritium unit.

Glossary

Glossary 77
Photograph by Alan M. Cressler, USGS

A basaltic-rock aquifer is actively being formed 

during a December 2011 eruption by the Kilauea 

Volcano, Hawaii Volcanoes National Park, Hawaii.
Appendixes

Appendix 1. NAWQA Groundwater Sampling Networks in 

the Columbia Plateau, Snake River Plain, and Oahu Aquifers

 How does land use affect groundwater quality? How does water quality change as 

it moves through an aquifer? What is the quality of the drinking-water resource? NAWQA 

groundwater assessments include different types of studies, specifically designed to answer 

questions such as these. 

•	 Land-use studies sampled water from water-table monitoring wells and (or) domestic 

wells installed in agricultural lands to assess the effects of agricultural land use on the 

quality of the underlying groundwater. 

•	 Major aquifer studies provide a broad overview of the quality of the aquifer system used 

for drinking-water supply. Most of the wells sampled were domestic or public-supply 

wells that were distributed across a large area in a mixture of land uses. 

•	 Special studies were designed to assess groundwater quality in particular areas of 

concern, or to sample for particular constituents of concern using laboratory methods 

that report concentrations much lower than standard laboratory methods. 

 Each study sampled water from a network of 10 to as many as 65 wells. Results of these 

studies were reinforced by locating some of the studies within the boundaries of larger studies. 

For example, the area sampled by the special studies was within the area sampled by the landuse studies, which in turn was within the area sampled by the major aquifer studies.

Appendix 1 79
Table A1–1. NAWQA groundwater sampling studies conducted in the Columbia Plateau and Snake River Plain and on Oahu.

[*, resampled in NAWQA Cycle II; —, not applicable or data insufficient to compute value; Abbreviations: DOH, Department of Health; EDB, ethylene 

dibromide; DBCP, dibromochloropropane; mg/L, milligrams per liter; CFC, chlorofluorocarbon; SF6, sulfur hexafluoride; ccpt, Central Columbia Plateau; 

usnk, Upper Snake River Basin; lus; land-use study; sus, study-unit (major aquifer) survey; spc, special study; the first four letters in the NAWQA groundwater

study name abbreviation indicate the study area, the next three letters indicate if the network is a lus, sus, or spc, and the remaining letters are unique identifiers]

Groundwater 

study name

used in

this report

NAWQA

well network 

type

What data were collected and why 

NAWQA 

groundwater 

study name

abbreviation

Type of sites sampled

Columbia Plateau, Washington 

Columbia public

supply*

Major aquifer 

study

Occurrence and distribution of chemicals in major aquifers. 

Major ions, nutrients, 85 pesticides, 60 volatile organic 

compounds, dissolved organic carbon, and radon. USGS 

collected, randomly selected wells.

ccptsus1b* Public-supply wells 

Estimate risks of pesticide detection. Data included 

47 pesticides. DOH collected pesticide kit. These well 

networks were not sampled for nitrate. 

ccptsus1a Public-supply wells 

ccptsus1c Public-supply wells with 

nitrate concentrations 

greater than 2 mg/L 

Palouse area Agricultural landuse study—

dryland grains

Describe effects of agricultural land use on shallow

groundwater in the Palouse subunit. Data included 

major ions, nutrients, 85 pesticides, 60 volatile organic 

compounds, dissolved organic carbon, and radon. Wells 

were generally next to fields in the road right-of-way.

ccptlusag1a Shallow domestic wells 

ccptlusag1b Very shallow monitoring 

wells

Quincy-Pasco 

row crops*

Agricultural landuse study—

irrigated row 

crops

Describe effects of agricultural land use on shallow 

groundwater in the Quincy-Pasco subunit. Data same 

as above. Wells were generally within 100 feet of 

row-cropped fields.

ccptlusag2a Shallow domestic wells 

ccptlusag2b* Very shallow monitoring 

wells

Quincy-Pasco 

orchards*

Agricultural landuse study—

orchards 

Describe effects of agricultural land use on shallow groundwater in the Quincy-Pasco subunit. Data same as above. 

Wells were generally within 100 feet of orchards/vineyards.

ccptlusor1a Shallow domestic wells 

ccptlusor1b* Very shallow monitoring 

wells

Quincy-Pasco 

fumigant study

Special study— 

EDB synoptic

Fumigants EDB and DBCP at low reporting levels. ccptspcb2 Shallow domestic wells 

Snake River Plain and Upper Snake River Basin, Idaho and Wyoming 

Snake River 

Plain

Major aquifer 

study

Occurrence and distribution of chemicals in major 

aquifers. Major ions, nutrients, 87 pesticides, volatile 

organic compounds, and radon. Data collected in 

cooperation with the Idaho Statewide Ground-Water 

Monitoring Program.

usnksus1 Domestic wells 

Tributary

valleys

usnksus2 Domestic wells 

Jackson valley usnksus3 Domestic and publicsupply wells

Minidoka area Agricultural 

land-use 

study— 

irrigated 

row crops

Major ions, nutrients, 87 pesticides, volatile organic 

compounds, and radon. Data collected in cooperation 

with the Idaho Statewide Ground-Water Monitoring 

Program.

usnkluscr1 Domestic wells 

A&B area* usnkluscr2 Domestic wells 

Jerome/Gooding 

area*

usnkluscr3 Domestic wells 

Eden area usnkluscr4 Domestic wells 

Oahu, Hawaii 

Oahu public 

supply

Major aquifer 

study

Occurrence and distribution of chemicals in major aquifers. Major ions, nutrients, trace elements, 87 pesticides, 

volatile organic compounds, and radon. CFC and tritiumhelium dates.

oahusus1 Public-supply wells 

Oahu, groundwater near 

water table

Special study— 

young groundwater in 

irrigationrecharge layer

Occurrence and distribution of chemicals in irrigationrecharge layer near the water table. Major ions, nutrients, 

trace elements, 87 pesticides, and volatile organic 

compounds. CFC, SF6 , and tritium-helium dates.

oahuspcg1 Monitoring wells 

80 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Appendixes

Table A1–1. NAWQA groundwater sampling studies conducted in the Columbia Plateau and Snake River Plain and on Oahu.—Continued

[*, resampled in NAWQA Cycle II; —, not applicable or data insufficient to compute value; Abbreviations: DOH, Department of Health; EDB, ethylene 

dibromide; DBCP, dibromochloropropane; mg/L, milligrams per liter; CFC, chlorofluorocarbon; SF6, sulfur hexafluoride; ccpt, Central Columbia Plateau; 

usnk, Upper Snake River Basin; lus; land-use study; sus, study-unit (major aquifer) survey; spc, special study; the first four letters in the NAWQA groundwater

study name abbreviation indicate the study area, the next three letters indicate if the network is a lus, sus, or spc, and the remaining letters are unique identifiers]

Groundwater 

study name 

used in

this report

Number

of sites 

Aquifer

lithology

Median 

well depth 

(feet)

Median

depth

to water

(feet)

Sampling

frequency

Predominant land use

or crop type

Columbia Plateau, Washington

Columbia public

supply*

43 Basalt 175 — Once in 1994; resampled 

in 2002

Mixed: agriculture, rangeland, and urban

65 Basalt 200 — Once in 1994

31 Basalt 185 — Once in 1994

Palouse area 18 Basalt 102 — Once in 1993–94 Agriculture (mostly wheat 

and small grains)

10 Loess (silt) 37 14 Once in 1993–94 

Quincy-Pasco 

row crops*

30 Sand, gravel, basalt 134 — Once in 1993–95 Agriculture (mostly 

potatoes and corn)

19 Sand, gravel 38 17 Once in 1993–95; 

resampled in 2002

Quincy-Pasco 

orchards*

18 Sand, gravel, basalt 165 55 Once in 1994–95 Agriculture (orchards 

and vineyards) 

22 Sand, gravel 29 13 Once in 1994–95; 

resampled in 2002

Quincy-Pasco 

fumigant study

11 Sand, gravel — — Once in 1994 Agriculture (including 

potatoes in rotation)

Snake River Plain and Upper Snake River Basin, Idaho and Wyoming

Snake River 

Plain

43 Basalt 260 — Once in 1994 Mixed: agriculture and 

rangeland

Tributary

valleys

38 Alluvium 204 — Once in 1995 Mixed: agriculture and 

rangeland

Jackson valley 20 Alluvium 101 7 Once in 1995 Mixed: forest and 

rangeland

Minidoka area 29 Sand, gravel 34 8 Once in 1993 Agriculture (mixed crops)

A&B area* 31 Basalt 228 178 Once in 1993; resampled 

in 2005

Agriculture (mixed crops)

Jerome/Gooding 

area*

30 Basalt 200 149 Once in 1994; resampled 

in 2005

Agriculture (mixed crops)

Eden area 15 Basalt 360 292 Once in 1995 Agriculture (mixed crops)

Oahu, Hawaii

Oahu public 

supply

30 Basalt 512 320 Once in 2000 Mixed: forest, urban, 

and agriculture 

(sugarcane, pineapple, 

mixed crops)

Oahu, groundwater near 

water table

15 Basalt 693 339 Once in 2001 Mixed: forest, urban, 

and agriculture 

(sugarcane, pineapple, 

mixed crops)

Appendix 1 81
OREGON IDAHO

WASHINGTON

Columbia

Plateau

120° 119° 118° 117°

48°

47°

Base modified from U.S. Geological Survey 1:2,000,000-scale digital data (2005), NAD 83 datum

Albers Equal Area Conic projection, standard parallels 29°30'N and 45°30'N, central meridian 119°W

0 10 20 30 40 50 MILES

0 10 20 30 40 50 KILOMETERS

EXPLANATION

Palouse area, agricultural lands—Dryland grains (ccptlusag1, see table A1–1)

Quincy-Pasco area, agricultural lands—Irrigated row crops (ccptlusag2) 

and orchards (ccptlusor1)

NAWQA Central Columbia Plateau study area—Columbia public supply, 

mixed land uses (the entire Central Columbia Plateau study area (ccptsus1))

Well sampling groundwater underlying 

Appendix 1-1

Mixed land uses, public-supply wells (ccptsus1)

Agricultural lands, Palouse area, domestic wells (ccptlusag1a) and 

monitoring wells (ccptlusag1b)

Agricultural lands, Quincy-Pasco row crops, domestic wells (ccptlusag2a) and 

monitoring wells (ccptlusag2b)

Agricultural lands, Quincy-Pasco orchards, domestic wells (ccptlusor1a) and 

monitoring wells (ccptlusor1b)

Agricultural lands, Quincy-Pasco fumigant study, domestic wells (ccptspcb2)

Figure A1–1. Location of all sampled wells in the Columbia Plateau.

82 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Appendixes

114° 112° 110°

44°

43°

42°

0 20 40 60 80 MILES

0 20 40 60 80 KILOMETERS

EXPLANATION

Snake River Plain, mixed land uses (usnksus1)

Tributary valleys, mixed land uses (usnksus2)

Jackson valley, mixed land uses (usnksus3)

Minidoka area, agricultural lands—Irrigated row crops, 

mostly surface-water irrigation (usnkluscr1)

A&B area, agricultural lands—Irrigated row crops, 

mostly groundwater irrigation (usnkluscr2)

Jerome/Gooding area, agricultural lands—Irrigated row crops, 

mostly surface-water irrigation (usnkluscr3)

Eden area, agricultural lands—Irrigated row crops, 

mostly surface-water irrigation (usnkluscr4)

NAWQA Upper Snake River Basin study area—Eastern Snake River Plain and 

tributary drainage areas

Wells sampling groundwater underlying mixed land uses—Domestic and public supply 

Wells sampling groundwater underlying agricultural lands—Domestic 

Base modified from U.S. Geological Survey 1:2,000,000-scale digital data (2005), NAD 83 datum

Albers Equal Area Conic projection, standard parallels 29°30'N and 45°30'N, central meridian 113°W

IDAHO

MONTANA WYOMING

NEVADA UTAH

Snake

River Plain

Appendix 1-2

River

Snake

Figure A1–2. Location of all sampled wells in the Snake River Plain.

Appendix 1 83
Appendix 1C

158°10' 158° 157°50' 157°40'

21°40'

21°30'

21°20'

Base modified from U.S. Geological Survey 1:24,000-scale digital data (2003), NAD 83 datum

Albers Equal Area Conic projection, standard parallels 8°N and 18°N, central meridian 157°W

0 3 6 9 12 MILES

0 3 6 9 12 KILOMETERS

EXPLANATION

NAWQA Oahu study area—Mixed land uses (oahusus1)

Wells sampling groundwater underlying mixed land uses

 Public supply (oahusus1)

 Groundwater near water table (oahuspcg1)

Figure A1–3. Location of all sampled wells on Oahu.

84 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Appendixes

Appendix 2. Groundwater-Quality Properties and Constituents

Measured, a Summary of Data, and Complete Data Archive for 

1993–2005 

Groundwater­quality properties and constituents measured and a summary of data for 1993– 2005, 

including laboratory reporting levels and human­health benchmarks for drinking water, are presented 

only online. The data summary and a complete data archive are available for download at http://pubs.

usgs.gov/circ/1359/.

Appendix 2 85
This appendix shows graphical comparisons of chemical concentrations (1991 to 2010) for atrazine, 1,2-dichloropropane 

(DCP), and nitrate in groundwater in selected Principal Aquifers of the United States. For each constituent, the 

concentration data are grouped according to two land uses: agricultural land use and mixed land uses. For each land use, 

the aquifers also are grouped according to aquifer lithology: basalt and volcanics,** crystalline, carbonate, sandstone, 

semiconsolidated sand and gravel, glacial unconsolidated sand and gravel, and unconsolidated sand and gravel 

(nonglacial). Data for a particular compound were not plotted if there were fewer than 10 samples for a particular well 

network in a Principal Aquifer; not all Principal Aquifers for which data were available are shown. Note that analytical 

detection limits varied among the constituents and that the number of samples for a constituent can vary greatly 

between Principal Aquifers. The data used in this appendix and boxplots for additional constituents are available at 

http://pubs.usgs.gov/circ/1360/.

** Note: Two of the Principal Aquifers in this group include limited samples from basin-fill aquifers within the extent of the basaltic aquifer. 

Principal Aquifer (number of samples)

10th

percentile

25th

percentile

75th

percentile

Median

(50th percentile)

Lithology

Basalt and volcanics** Crystalline Carbonate Sandstone Semiconsolidated 

 sand and gravel

Glacial unconsolidated sand and gravel Unconsolidated sand and gravel (nonglacial)

MCL Maximum Contaminant Level

U.S. Environmental Protection Agency

 contaminant levels for drinking water 

(NA) Wells of this type were not 

 sampled for this constituent

90th

percentile

Data above

90th percentile

Data below

10th percentile*

SMCL Secondary Maximum Contaminant Level

PMCL Proposed Maximum Contaminant Level

AMCL Proposed Alternative Maximum Contaminant Level

* Data below laboratory reporting limits are not shown.

86 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005

Appendix 3. Groundwater Quality of the Columbia Plateau, Snake River

Plain, and Oahu Aquifers in a National Context
Appendixes

Figure A2-1, cir1359_append2_National_atrazine_boxplot_20130716_wevo.ai

Atrazine concentration, in micrograms per liter

0.001 0.01 0.1 1 10 0.001 0.01 0.1 1 10

Principal Aquifer Agricultural land-use wells Mixed land-use wells

MCL=3 μg/L MCL=3 μg/L

Surficial (135)

Mississippi River Valley alluvial (NA)

High Plains (85)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (19)

Rio Grande (91)

Central Valley (140)

California Coastal Basin (NA)

Basin and Range basin fill (37)

Glacial–West (25)

Glacial–West Central (153)

Glacial–Central (281)

Glacial–East (54)

Texas coastal uplands (NA)

Northern Atlantic Coastal Plain (83)

Mississippi embayment (NA)

Valley and Ridge siliciclastic (24)

Early Mesozoic (NA)

Denver Basin (11)

Cambrian-Ordovician (NA)

Valley and Ridge carbonate (93)

Piedmont and Blue Ridge carbonate (30)

Upper Floridan (6)

Castle Hayne (NA)

Piedmont and Blue Ridge crystalline (NA)

New England crystalline (NA)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (110)

Columbia Plateau basin fill–basalt rock (128)

(30)

(53)

(339)

(NA)

(NA)

(29)

(63)

(132)

(278)

(64)

(134)

(199)

(138)

(65)

(118)

(73)

(58)

(73)

(59)

(140)

(30)

(NA)

(150)

(30)

(169)

(115)

(29)

(98)

 (43)

Atrazine

0.1 1 10 100 0.1 1 10 100

FiA22i1359di2NtilDCPblt20130716i

DCP concentration, in micrograms per liter

Agricultural land-use wells Mixed land-use wells

MCL=5 μg/L MCL=5 μg/L

Principal Aquifer

Surficial (57)

Mississippi River Valley alluvial (NA)

High Plains (NA)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (NA)

Rio Grande (55)

Central Valley (133)

California Coastal Basin (NA)

Basin and Range basin fill (37)

Glacial–West (25)

Glacial–West Central (90)

Glacial–Central (126)

Glacial–East (28)

Texas coastal uplands (NA)

Northern Atlantic Coastal Plain (36)

Mississippi embayment (NA)

Valley and Ridge siliciclastic (NA)

Early Mesozoic (NA)

Denver Basin (NA)

Cambrian-Ordovician (NA)

Valley and Ridge carbonate (46)

Piedmont and Blue Ridge carbonate (30)

Upper Floridan (4)

Castle Hayne (NA)

Piedmont and Blue Ridge crystalline (NA)

New England crystalline (NA)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (104)

Columbia Plateau basin fill–basalt rock (119)

(30)

(54)

(338)

(NA)

(NA)

(29)

(63)

(133)

(265)

(63)

(81)

(175)

(138)

(65)

(116)

(74)

(59)

(51)

(59)

(119)

(30)

(NA)

(150)

(30)

(125)

(117)

(30)

(25)

 (41)

1,2-dichloropropane (DCP)

Appendix 3 87
0.01 0.1 1 10 100 0.01 0.1 1 10 100

Nitrate concentration, in milligrams per liter as nitrogen

Principal Aquifer Agricultural land-use wells Mixed land-use wells

MCL=10 mg/L

MCL=10 mg/L

Surficial (138)

Mississippi River Valley alluvial (NA)

High Plains (85)

Alluvial–Pleistocene terrace deposits (NA)

Alluvial–Denver Basin (19)

Rio Grande (119)

Central Valley (140)

California Coastal Basin (NA)

Basin and Range basin fill (37)

Glacial–West (25)

Glacial–West Central (154)

Glacial–Central (282)

Glacial–East (54)

Texas coastal uplands (NA)

Northern Atlantic Coastal Plain (83)

Mississippi embayment (NA)

Valley and Ridge siliciclastic (23)

Early Mesozoic (NA)

Denver Basin (11)

Cambrian-Ordovician (NA)

Valley and Ridge carbonate (92)

Piedmont and Blue Ridge carbonate (30)

Upper Floridan (6)

Castle Hayne (NA)

Piedmont and Blue Ridge crystalline (NA)

New England crystalline (NA)

Hawaiian volcanic (NA)

Snake River Plain basin fill–basalt rock (111)

Columbia Plateau basin fill–basalt rock (129)

(30)

(54)

(338)

(NA)

(NA)

(30)

(62)

(132)

(271)

(64)

(140)

(199)

(137)

(65)

(118)

(74)

(59)

(74)

(79)

(138)

(30)

(NA)

(153)

(30)

(169)

(117)

(30)

(101)

 (43)

Nitrite plus Nitrate (nitrate)

88 Groundwater Quality in the Columbia Plateau, Snake River Plain, and Oahu Aquifers, 1993–2005
Prepared by the USGS Science Publishing Network

Publishing Service Centers

Edited by Kay P. Naugle, Raleigh

Illustrations by Luis E. Menoyo, Tacoma

 and Caryl J. Wipperfurth, Raleigh

Layout by Caryl J. Wipperfurth

For more information concerning this report, contact:

Chief, National Water-Quality Assessment Program

U.S. Geological Survey

413 National Center

12201 Sunrise Valley Drive 

Reston, VA 20192

http://water.usgs.gov/nawqa/
Snake River Plain

basin-fill and

basaltic-rock

aquifers

Columbia Plateau

basin-fill and

basaltic-rock

aquifers

Hawaiian

volcanic-rock

aquifers

Printed on recycled paper

ISSN 1067-084X (print)

ISSN 2330-5703 (online)

http://dx.doi.org/10.3133/cir1359